You can extending Universal Theme by attaching handlers to its events.

1. theme42ready is triggered after the initialization of Universal Theme:

   apex.jQuery(window).on('theme42ready', function() {
       console.log('Do something after UI elements are rendered on the page.');
   });

2. theme42layoutchanged is triggered when left navigation menu is expanded or collapsed.

   apex.jQuery("#t_TreeNav").on('theme42layoutchanged', function(event, obj) {
       console.log('Left menu action: ' + obj.action);   // obj.action will return expand or collapse
   });

3. apexwindowresized is triggered by the browser window resize or the maximize/restore action of a region.

   apex.jQuery(window).on('apexwindowresized', function() {
       console.log('Window resized.');
   });