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/sessionMode.js

@@ -15,6 +15,7 @@ const _modes = {
     'restrictive': {
         parentMode: null,
         stylesheetName: 'gnome-shell.css',
+        themeResourceName: 'gnome-shell-theme.gresource',
         hasOverview: false,
         showCalendarEvents: false,
         allowSettings: false,