Set compact dark theme in firefox

programs/firefox.nix

@@ -4,6 +4,12 @@
   programs.firefox = {
     enable = true;
 
+    profiles.default = {
+      settings = {
+        "extensions.activeThemeID" = "firefox-compact-dark@mozilla.org";
+      };
+    };
+
     policies = {
       # From https://discourse.nixos.org/t/declare-firefox-extensions-and-settings/36265/17
       ExtensionSettings = with builtins;