jQuery event that triggers after CSS is loaded?

Rather than creating a new link element and appending it to the head, you could retrieve the contents of the stylesheet with an AJAX call, and insert it into an inline style block. That way, you can use jQuery’s ‘complete’ callback to fire off your check.

$('#theme-selector a').click(function(){
  var path = $(this).attr('href');
  $.get(path, function(response){
   //Check if the user theme element is in place - if not, create it.
   if (!$('#userTheme').length) $('head').append('<style id="userTheme"></style>');

   //populate the theme element with the new style (replace the old one if necessary)
   $('#userTheme').text(response);

  //Check whatever you want about the new style:
  alert($('body').css('background-color'));
  });
});

I haven’t actually tested this code, so there may be some syntax-y errors, but the logic should be sound enough.