diff --git a/modules/misc/gtk.nix b/modules/misc/gtk.nix
index 8fe80910a..d70efdd6f 100644
--- a/modules/misc/gtk.nix
+++ b/modules/misc/gtk.nix
@@ -416,20 +416,9 @@ in
       })
 
       # GTK4 Configuration
-      (mkIf cfg4.enable (
-        let
-          gtk4Css =
-            lib.optionalString (cfg4.theme != null && cfg4.theme.package != null) ''
-              /**
-               * GTK 4 reads the theme configured by gtk-theme-name, but ignores it.
-               * It does however respect user CSS, so import the theme from here.
-              **/
-              @import url("file://${cfg4.theme.package}/share/themes/${cfg4.theme.name}/gtk-4.0/gtk.css");
-            ''
-            + cfg4.extraCss;
-        in
-        {
-          xdg.configFile."gtk-4.0/settings.ini" = {
+      (mkIf cfg4.enable {
+        xdg.configFile = {
+          "gtk-4.0/settings.ini" = {
             text = toIni {
               Settings =
                 mkGtkSettings {
@@ -443,11 +432,21 @@ in
                 // cfg4.extraConfig;
             };
           };
-          xdg.configFile."gtk-4.0/gtk.css" = mkIf (gtk4Css != "") {
-            text = gtk4Css;
-          };
-        }
-      ))
+          "gtk-4.0/gtk.css" =
+            mkIf (cfg4.extraCss != "" || (cfg4.theme != null && cfg4.theme.package != null))
+              {
+                text =
+                  lib.optionalString (cfg4.theme != null && cfg4.theme.package != null) ''
+                    /**
+                     * GTK 4 reads the theme configured by gtk-theme-name, but ignores it.
+                     * It does however respect user CSS, so import the theme from here.
+                    **/
+                    @import url("file://${cfg4.theme.package}/share/themes/${cfg4.theme.name}/gtk-4.0/gtk.css");
+                  ''
+                  + cfg4.extraCss;
+              };
+        };
+      })
     ]
   );
 }
diff --git a/tests/modules/misc/gtk/default.nix b/tests/modules/misc/gtk/default.nix
index c43165ca9..016658a69 100644
--- a/tests/modules/misc/gtk/default.nix
+++ b/tests/modules/misc/gtk/default.nix
@@ -4,13 +4,14 @@
   gtk-selective-enable = ./gtk-selective-enable.nix;
 
   # GTK2
-  gtk2-basic-config = ./gtk2-basic-config.nix;
-  gtk2-config-file-location = ./gtk2-config-file-location.nix;
+  gtk2-basic-config = ./gtk2/gtk2-basic-config.nix;
+  gtk2-config-file-location = ./gtk2/gtk2-config-file-location.nix;
 
   # GTK3
-  gtk3-basic-settings = ./gtk3-basic-settings.nix;
+  gtk3-basic-settings = ./gtk3/gtk3-basic-settings.nix;
 
   # GTK4
-  gtk4-basic-settings = ./gtk4-basic-settings.nix;
-  gtk4-theme-css-injection = ./gtk4-theme-css-injection.nix;
+  gtk4-basic-settings = ./gtk4/gtk4-basic-settings.nix;
+  gtk4-theme-css-injection = ./gtk4/gtk4-theme-css-injection.nix;
+  gtk4-no-theme-css-injection = ./gtk4/gtk4-no-theme-css-injection.nix;
 }
diff --git a/tests/modules/misc/gtk/gtk-global-inheritance-gtk2-expected.conf b/tests/modules/misc/gtk/gtk-global-inheritance-gtk2-expected.conf
new file mode 100644
index 000000000..4844a49b8
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk-global-inheritance-gtk2-expected.conf
@@ -0,0 +1,6 @@
+gtk-cursor-theme-name = "Adwaita"
+gtk-cursor-theme-size = 24
+gtk-font-name = "Ubuntu 12"
+gtk-icon-theme-name = "Adwaita"
+gtk-theme-name = "Adwaita-dark"
+
diff --git a/tests/modules/misc/gtk/gtk-global-inheritance-gtk3-expected.ini b/tests/modules/misc/gtk/gtk-global-inheritance-gtk3-expected.ini
new file mode 100644
index 000000000..5ab5e63c4
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk-global-inheritance-gtk3-expected.ini
@@ -0,0 +1,6 @@
+[Settings]
+gtk-cursor-theme-name=Adwaita
+gtk-cursor-theme-size=24
+gtk-font-name=Ubuntu 12
+gtk-icon-theme-name=Adwaita
+gtk-theme-name=Adwaita-dark
diff --git a/tests/modules/misc/gtk/gtk-global-inheritance-gtk4-css-expected.css b/tests/modules/misc/gtk/gtk-global-inheritance-gtk4-css-expected.css
new file mode 100644
index 000000000..a8f04a5ca
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk-global-inheritance-gtk4-css-expected.css
@@ -0,0 +1,5 @@
+/**
+ * GTK 4 reads the theme configured by gtk-theme-name, but ignores it.
+ * It does however respect user CSS, so import the theme from here.
+**/
+@import url("file://@gnome-themes-extra@/share/themes/Adwaita-dark/gtk-4.0/gtk.css");
diff --git a/tests/modules/misc/gtk/gtk-global-inheritance-gtk4-expected.ini b/tests/modules/misc/gtk/gtk-global-inheritance-gtk4-expected.ini
new file mode 100644
index 000000000..5ab5e63c4
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk-global-inheritance-gtk4-expected.ini
@@ -0,0 +1,6 @@
+[Settings]
+gtk-cursor-theme-name=Adwaita
+gtk-cursor-theme-size=24
+gtk-font-name=Ubuntu 12
+gtk-icon-theme-name=Adwaita
+gtk-theme-name=Adwaita-dark
diff --git a/tests/modules/misc/gtk/gtk-global-inheritance.nix b/tests/modules/misc/gtk/gtk-global-inheritance.nix
index 444522046..8ad2edc22 100644
--- a/tests/modules/misc/gtk/gtk-global-inheritance.nix
+++ b/tests/modules/misc/gtk/gtk-global-inheritance.nix
@@ -1,6 +1,5 @@
 { pkgs, ... }:
 {
-  # Test that global settings are inherited by all GTK versions
   gtk = {
     enable = true;
     font = {
@@ -25,33 +24,21 @@
   nmt.script = ''
     # Check GTK2 inherits global settings
     assertFileExists home-files/.gtkrc-2.0
-    assertFileRegex home-files/.gtkrc-2.0 'gtk-theme-name = "Adwaita-dark"'
-    assertFileRegex home-files/.gtkrc-2.0 'gtk-font-name = "Ubuntu 12"'
-    assertFileRegex home-files/.gtkrc-2.0 'gtk-icon-theme-name = "Adwaita"'
-    assertFileRegex home-files/.gtkrc-2.0 'gtk-cursor-theme-name = "Adwaita"'
-    assertFileRegex home-files/.gtkrc-2.0 'gtk-cursor-theme-size = 24'
+    assertFileContent home-files/.gtkrc-2.0 \
+      ${./gtk-global-inheritance-gtk2-expected.conf}
 
     # Check GTK3 inherits global settings
     assertFileExists home-files/.config/gtk-3.0/settings.ini
-    assertFileRegex home-files/.config/gtk-3.0/settings.ini 'gtk-theme-name=Adwaita-dark'
-    assertFileRegex home-files/.config/gtk-3.0/settings.ini 'gtk-font-name=Ubuntu 12'
-    assertFileRegex home-files/.config/gtk-3.0/settings.ini 'gtk-icon-theme-name=Adwaita'
-    assertFileRegex home-files/.config/gtk-3.0/settings.ini 'gtk-cursor-theme-name=Adwaita'
-    assertFileRegex home-files/.config/gtk-3.0/settings.ini 'gtk-cursor-theme-size=24'
+    assertFileContent home-files/.config/gtk-3.0/settings.ini \
+      ${./gtk-global-inheritance-gtk3-expected.ini}
 
     # Check GTK4 inherits global settings
     assertFileExists home-files/.config/gtk-4.0/settings.ini
-    assertFileRegex home-files/.config/gtk-4.0/settings.ini 'gtk-theme-name=Adwaita-dark'
-    assertFileRegex home-files/.config/gtk-4.0/settings.ini 'gtk-font-name=Ubuntu 12'
-    assertFileRegex home-files/.config/gtk-4.0/settings.ini 'gtk-icon-theme-name=Adwaita'
-    assertFileRegex home-files/.config/gtk-4.0/settings.ini 'gtk-cursor-theme-name=Adwaita'
-    assertFileRegex home-files/.config/gtk-4.0/settings.ini 'gtk-cursor-theme-size=24'
+    assertFileContent home-files/.config/gtk-4.0/settings.ini \
+      ${./gtk-global-inheritance-gtk4-expected.ini}
 
     # Check GTK4 CSS with theme import
     assertFileExists home-files/.config/gtk-4.0/gtk.css
-    assertFileRegex home-files/.config/gtk-4.0/gtk.css '@import url("file://.*/share/themes/Adwaita-dark/gtk-4.0/gtk.css")'
-
-    # Check packages are installed
-    # Package installation verified by home-manager
+    assertFileContent home-files/.config/gtk-4.0/gtk.css ${./gtk-global-inheritance-gtk4-css-expected.css}
   '';
 }
diff --git a/tests/modules/misc/gtk/gtk-per-version-override-gtk2-expected.conf b/tests/modules/misc/gtk/gtk-per-version-override-gtk2-expected.conf
new file mode 100644
index 000000000..28e4ce54a
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk-per-version-override-gtk2-expected.conf
@@ -0,0 +1,5 @@
+gtk-cursor-theme-name = "Global-Cursor"
+gtk-font-name = "GTK2-Font 11"
+gtk-icon-theme-name = "Global-Icons"
+gtk-theme-name = "GTK2-Theme"
+gtk-can-change-accels = 1
diff --git a/tests/modules/misc/gtk/gtk-per-version-override-gtk3-css-expected.css b/tests/modules/misc/gtk/gtk-per-version-override-gtk3-css-expected.css
new file mode 100644
index 000000000..50581c0b7
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk-per-version-override-gtk3-css-expected.css
@@ -0,0 +1 @@
+window { border: 1px solid red; }
\ No newline at end of file
diff --git a/tests/modules/misc/gtk/gtk-per-version-override-gtk3-expected.ini b/tests/modules/misc/gtk/gtk-per-version-override-gtk3-expected.ini
new file mode 100644
index 000000000..60faf9172
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk-per-version-override-gtk3-expected.ini
@@ -0,0 +1,6 @@
+[Settings]
+gtk-cursor-theme-name=GTK3-Cursor
+gtk-font-name=Global Font 11
+gtk-icon-theme-name=GTK3-Icons
+gtk-recent-files-limit=10
+gtk-theme-name=Global-Theme
diff --git a/tests/modules/misc/gtk/gtk-per-version-override-gtk4-css-expected.css b/tests/modules/misc/gtk/gtk-per-version-override-gtk4-css-expected.css
new file mode 100644
index 000000000..5c6a5cd74
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk-per-version-override-gtk4-css-expected.css
@@ -0,0 +1 @@
+window { border: 2px solid blue; }
\ No newline at end of file
diff --git a/tests/modules/misc/gtk/gtk-per-version-override-gtk4-expected.ini b/tests/modules/misc/gtk/gtk-per-version-override-gtk4-expected.ini
new file mode 100644
index 000000000..25b6ea043
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk-per-version-override-gtk4-expected.ini
@@ -0,0 +1,6 @@
+[Settings]
+gtk-cursor-theme-name=Global-Cursor
+gtk-font-name=GTK4-Font 11
+gtk-icon-theme-name=Global-Icons
+gtk-recent-files-limit=20
+gtk-theme-name=Global-Theme
diff --git a/tests/modules/misc/gtk/gtk-per-version-override.nix b/tests/modules/misc/gtk/gtk-per-version-override.nix
index 75f54a911..0273de709 100644
--- a/tests/modules/misc/gtk/gtk-per-version-override.nix
+++ b/tests/modules/misc/gtk/gtk-per-version-override.nix
@@ -1,5 +1,4 @@
 {
-  # Test that per-version settings override global defaults
   gtk = {
     enable = true;
     # Global defaults
@@ -35,26 +34,24 @@
 
   nmt.script = ''
     # GTK2 should use overridden font/theme, global icons/cursor
-    assertFileRegex home-files/.gtkrc-2.0 'gtk-font-name = "GTK2-Font 11"'
-    assertFileRegex home-files/.gtkrc-2.0 'gtk-theme-name = "GTK2-Theme"'
-    assertFileRegex home-files/.gtkrc-2.0 'gtk-icon-theme-name = "Global-Icons"'
-    assertFileRegex home-files/.gtkrc-2.0 'gtk-cursor-theme-name = "Global-Cursor"'
-    assertFileRegex home-files/.gtkrc-2.0 'gtk-can-change-accels = 1'
+    assertFileExists home-files/.gtkrc-2.0
+    assertFileContent home-files/.gtkrc-2.0 \
+      ${./gtk-per-version-override-gtk2-expected.conf}
 
     # GTK3 should use global font/theme, overridden icons/cursor
-    assertFileRegex home-files/.config/gtk-3.0/settings.ini 'gtk-font-name=Global Font 11'
-    assertFileRegex home-files/.config/gtk-3.0/settings.ini 'gtk-theme-name=Global-Theme'
-    assertFileRegex home-files/.config/gtk-3.0/settings.ini 'gtk-icon-theme-name=GTK3-Icons'
-    assertFileRegex home-files/.config/gtk-3.0/settings.ini 'gtk-cursor-theme-name=GTK3-Cursor'
-    assertFileRegex home-files/.config/gtk-3.0/settings.ini 'gtk-recent-files-limit=10'
-    assertFileRegex home-files/.config/gtk-3.0/gtk.css 'window { border: 1px solid red; }'
+    assertFileExists home-files/.config/gtk-3.0/settings.ini
+    assertFileContent home-files/.config/gtk-3.0/settings.ini \
+      ${./gtk-per-version-override-gtk3-expected.ini}
+    assertFileExists home-files/.config/gtk-3.0/gtk.css
+    assertFileContent home-files/.config/gtk-3.0/gtk.css \
+      ${./gtk-per-version-override-gtk3-css-expected.css}
 
     # GTK4 should use overridden font, global theme/icons/cursor
-    assertFileRegex home-files/.config/gtk-4.0/settings.ini 'gtk-font-name=GTK4-Font 11'
-    assertFileRegex home-files/.config/gtk-4.0/settings.ini 'gtk-theme-name=Global-Theme'
-    assertFileRegex home-files/.config/gtk-4.0/settings.ini 'gtk-icon-theme-name=Global-Icons'
-    assertFileRegex home-files/.config/gtk-4.0/settings.ini 'gtk-cursor-theme-name=Global-Cursor'
-    assertFileRegex home-files/.config/gtk-4.0/settings.ini 'gtk-recent-files-limit=20'
-    assertFileRegex home-files/.config/gtk-4.0/gtk.css 'window { border: 2px solid blue; }'
+    assertFileExists home-files/.config/gtk-4.0/settings.ini
+    assertFileContent home-files/.config/gtk-4.0/settings.ini \
+      ${./gtk-per-version-override-gtk4-expected.ini}
+    assertFileExists home-files/.config/gtk-4.0/gtk.css
+    assertFileContent home-files/.config/gtk-4.0/gtk.css \
+      ${./gtk-per-version-override-gtk4-css-expected.css}
   '';
 }
diff --git a/tests/modules/misc/gtk/gtk-selective-enable-gtk3-expected.ini b/tests/modules/misc/gtk/gtk-selective-enable-gtk3-expected.ini
new file mode 100644
index 000000000..5b16ed39a
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk-selective-enable-gtk3-expected.ini
@@ -0,0 +1,2 @@
+[Settings]
+gtk-theme-name=Test-Theme
diff --git a/tests/modules/misc/gtk/gtk-selective-enable.nix b/tests/modules/misc/gtk/gtk-selective-enable.nix
index bc032a3b3..95bb66244 100644
--- a/tests/modules/misc/gtk/gtk-selective-enable.nix
+++ b/tests/modules/misc/gtk/gtk-selective-enable.nix
@@ -16,7 +16,8 @@
 
     # GTK3 should be configured
     assertFileExists home-files/.config/gtk-3.0/settings.ini
-    assertFileRegex home-files/.config/gtk-3.0/settings.ini 'gtk-theme-name=Test-Theme'
+    assertFileContent home-files/.config/gtk-3.0/settings.ini \
+      ${./gtk-selective-enable-gtk3-expected.ini}
 
     # GTK4 should not be configured
     assertPathNotExists home-files/.config/gtk-4.0/settings.ini
diff --git a/tests/modules/misc/gtk/gtk2-basic-config-expected.conf b/tests/modules/misc/gtk/gtk2/gtk2-basic-config-expected.conf
similarity index 100%
rename from tests/modules/misc/gtk/gtk2-basic-config-expected.conf
rename to tests/modules/misc/gtk/gtk2/gtk2-basic-config-expected.conf
diff --git a/tests/modules/misc/gtk/gtk2-basic-config.nix b/tests/modules/misc/gtk/gtk2/gtk2-basic-config.nix
similarity index 100%
rename from tests/modules/misc/gtk/gtk2-basic-config.nix
rename to tests/modules/misc/gtk/gtk2/gtk2-basic-config.nix
diff --git a/tests/modules/misc/gtk/gtk2-config-file-location.nix b/tests/modules/misc/gtk/gtk2/gtk2-config-file-location.nix
similarity index 100%
rename from tests/modules/misc/gtk/gtk2-config-file-location.nix
rename to tests/modules/misc/gtk/gtk2/gtk2-config-file-location.nix
diff --git a/tests/modules/misc/gtk/gtk3-basic-settings-expected.ini b/tests/modules/misc/gtk/gtk3/gtk3-basic-settings-expected.ini
similarity index 100%
rename from tests/modules/misc/gtk/gtk3-basic-settings-expected.ini
rename to tests/modules/misc/gtk/gtk3/gtk3-basic-settings-expected.ini
diff --git a/tests/modules/misc/gtk/gtk3-basic-settings.nix b/tests/modules/misc/gtk/gtk3/gtk3-basic-settings.nix
similarity index 100%
rename from tests/modules/misc/gtk/gtk3-basic-settings.nix
rename to tests/modules/misc/gtk/gtk3/gtk3-basic-settings.nix
diff --git a/tests/modules/misc/gtk/gtk4-theme-css-injection.nix b/tests/modules/misc/gtk/gtk4-theme-css-injection.nix
deleted file mode 100644
index 01d1dffd9..000000000
--- a/tests/modules/misc/gtk/gtk4-theme-css-injection.nix
+++ /dev/null
@@ -1,49 +0,0 @@
-{ pkgs, ... }:
-{
-  # Test GTK4 theme CSS injection with both theme and custom CSS
-  gtk = {
-    enable = true;
-
-    gtk4 = {
-      theme = {
-        name = "Adwaita-dark";
-        package = pkgs.gnome-themes-extra;
-      };
-      extraCss = ''
-        /* Custom user CSS */
-        window {
-          background-color: #2d2d2d;
-        }
-
-        button {
-          border-radius: 8px;
-        }
-      '';
-    };
-  };
-
-  nmt.script = ''
-    # GTK4 CSS should exist and contain both theme import and custom CSS
-    assertFileExists home-files/.config/gtk-4.0/gtk.css
-
-    # Check for theme import comment and URL
-    assertFileRegex home-files/.config/gtk-4.0/gtk.css '/\*\*'
-    assertFileRegex home-files/.config/gtk-4.0/gtk.css 'GTK 4 reads the theme configured by gtk-theme-name, but ignores it'
-    assertFileRegex home-files/.config/gtk-4.0/gtk.css '@import url("file://.*/share/themes/Adwaita-dark/gtk-4.0/gtk.css")'
-
-    # Check for custom CSS
-    assertFileRegex home-files/.config/gtk-4.0/gtk.css 'Custom user CSS'
-    assertFileRegex home-files/.config/gtk-4.0/gtk.css 'background-color: #2d2d2d'
-    assertFileRegex home-files/.config/gtk-4.0/gtk.css 'border-radius: 8px'
-
-    # Verify the theme import comes before custom CSS
-    css_content=$(cat home-files/.config/gtk-4.0/gtk.css)
-    import_line=$(echo "$css_content" | grep -n "@import" | cut -d: -f1)
-    custom_line=$(echo "$css_content" | grep -n "Custom user CSS" | cut -d: -f1)
-
-    if [ "$import_line" -ge "$custom_line" ]; then
-      echo "Theme import should come before custom CSS"
-      exit 1
-    fi
-  '';
-}
diff --git a/tests/modules/misc/gtk/gtk4/gtk4-basic-settings-expected.css b/tests/modules/misc/gtk/gtk4/gtk4-basic-settings-expected.css
new file mode 100644
index 000000000..7d27b3119
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk4/gtk4-basic-settings-expected.css
@@ -0,0 +1,5 @@
+/**
+ * GTK 4 reads the theme configured by gtk-theme-name, but ignores it.
+ * It does however respect user CSS, so import the theme from here.
+**/
+@import url("file://@catppuccin-gtk@/share/themes/catppuccin-macchiato-blue-standard/gtk-4.0/gtk.css");
diff --git a/tests/modules/misc/gtk/gtk4-basic-settings-expected.ini b/tests/modules/misc/gtk/gtk4/gtk4-basic-settings-expected.ini
similarity index 100%
rename from tests/modules/misc/gtk/gtk4-basic-settings-expected.ini
rename to tests/modules/misc/gtk/gtk4/gtk4-basic-settings-expected.ini
diff --git a/tests/modules/misc/gtk/gtk4-basic-settings.nix b/tests/modules/misc/gtk/gtk4/gtk4-basic-settings.nix
similarity index 86%
rename from tests/modules/misc/gtk/gtk4-basic-settings.nix
rename to tests/modules/misc/gtk/gtk4/gtk4-basic-settings.nix
index 6caa6fcd4..101cb0258 100644
--- a/tests/modules/misc/gtk/gtk4-basic-settings.nix
+++ b/tests/modules/misc/gtk/gtk4/gtk4-basic-settings.nix
@@ -20,10 +20,11 @@
     in
     ''
       assertFileExists ${gtk4Path}/settings.ini
-
       assertFileContent ${gtk4Path}/settings.ini \
         ${./gtk4-basic-settings-expected.ini}
 
       assertFileExists ${gtk4Path}/gtk.css
+      assertFileContent ${gtk4Path}/gtk.css \
+        ${./gtk4-basic-settings-expected.css}
     '';
 }
diff --git a/tests/modules/misc/gtk/gtk4/gtk4-no-theme-css-injection-expected.css b/tests/modules/misc/gtk/gtk4/gtk4-no-theme-css-injection-expected.css
new file mode 100644
index 000000000..1afa2c2d1
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk4/gtk4-no-theme-css-injection-expected.css
@@ -0,0 +1,8 @@
+/* Custom user CSS */
+window {
+  background-color: #2d2d2d;
+}
+
+button {
+  border-radius: 8px;
+}
diff --git a/tests/modules/misc/gtk/gtk4/gtk4-no-theme-css-injection.nix b/tests/modules/misc/gtk/gtk4/gtk4-no-theme-css-injection.nix
new file mode 100644
index 000000000..6cb3ce5d4
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk4/gtk4-no-theme-css-injection.nix
@@ -0,0 +1,25 @@
+{
+  # Test GTK4 theme CSS injection with both theme and custom CSS
+  gtk = {
+    enable = true;
+
+    gtk4 = {
+      extraCss = ''
+        /* Custom user CSS */
+        window {
+          background-color: #2d2d2d;
+        }
+
+        button {
+          border-radius: 8px;
+        }
+      '';
+    };
+  };
+
+  nmt.script = ''
+    # GTK4 CSS should exist and contain no theme import and only custom CSS
+    assertFileExists home-files/.config/gtk-4.0/gtk.css
+    assertFileContent home-files/.config/gtk-4.0/gtk.css ${./gtk4-no-theme-css-injection-expected.css}
+  '';
+}
diff --git a/tests/modules/misc/gtk/gtk4/gtk4-theme-css-injection-expected.css b/tests/modules/misc/gtk/gtk4/gtk4-theme-css-injection-expected.css
new file mode 100644
index 000000000..626fbe2e8
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk4/gtk4-theme-css-injection-expected.css
@@ -0,0 +1,13 @@
+/**
+ * GTK 4 reads the theme configured by gtk-theme-name, but ignores it.
+ * It does however respect user CSS, so import the theme from here.
+**/
+@import url("file://@gnome-themes-extra@/share/themes/Adwaita-dark/gtk-4.0/gtk.css");
+/* Custom user CSS */
+window {
+  background-color: #2d2d2d;
+}
+
+button {
+  border-radius: 8px;
+}
diff --git a/tests/modules/misc/gtk/gtk4/gtk4-theme-css-injection.nix b/tests/modules/misc/gtk/gtk4/gtk4-theme-css-injection.nix
new file mode 100644
index 000000000..e8dec1463
--- /dev/null
+++ b/tests/modules/misc/gtk/gtk4/gtk4-theme-css-injection.nix
@@ -0,0 +1,30 @@
+{ pkgs, ... }:
+{
+  # Test GTK4 theme CSS injection with both theme and custom CSS
+  gtk = {
+    enable = true;
+
+    gtk4 = {
+      theme = {
+        name = "Adwaita-dark";
+        package = pkgs.gnome-themes-extra;
+      };
+      extraCss = ''
+        /* Custom user CSS */
+        window {
+          background-color: #2d2d2d;
+        }
+
+        button {
+          border-radius: 8px;
+        }
+      '';
+    };
+  };
+
+  nmt.script = ''
+    # GTK4 CSS should exist and contain both theme import and custom CSS
+    assertFileExists home-files/.config/gtk-4.0/gtk.css
+    assertFileContent home-files/.config/gtk-4.0/gtk.css ${./gtk4-theme-css-injection-expected.css}
+  '';
+}