main: Get the theme resource name from sessionMode

Along the lines of `styleSheetName`, a session mode may want to provide its
own gresource file, so make this possible via a `themeResourceName` session
mode parameter, defaulted to gnome-shell-theme.gresource

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1063

js/ui/main.js

@@ -361,7 +361,8 @@ function reloadThemeResource() {
     if (_themeResource)
         _themeResource._unregister();
 
-    _themeResource = Gio.Resource.load('%s/gnome-shell-theme.gresource'.format(global.datadir));
+    _themeResource = Gio.Resource.load('%s/%s'.format(global.datadir,
+        sessionMode.themeResourceName));
     _themeResource._register();
 }