Split client specific and user css for firefox

.gitattributes

@@ -1,4 +1,4 @@
-home/programs/firefox/userContent.css filter=git-crypt diff=git-crypt
+home/programs/firefox/clientSpecific.css filter=git-crypt diff=git-crypt
 home/programs/ssh-match-blocks.nix filter=git-crypt diff=git-crypt
 home/files/composer/auth.json filter=git-crypt diff=git-crypt
 home/files/docker/config.json filter=git-crypt diff=git-crypt

home/programs/firefox.nix

@@ -263,7 +263,10 @@ in {
 
         "network.IDN_show_punycode" = true;
       };
-      userContent = builtins.readFile(./firefox/userContent.css);
+
+      # See: https://lists.w3.org/Archives/Public/www-style/2004Aug/0135
+      # See: https://developer.mozilla.org/en-US/docs/Web/CSS/@document
+      userContent = builtins.readFile(./firefox/userContent.css) + builtins.readFile(./firefox/clientSpecific.css);
     };
     toggl = {
       name = "toggl";