Cannot change CSS after CSS/JS aggregation
CSS/JS aggregation's purpose is to bundle all your non-external css/js files into a single file. This way instead of requesting multiple files and requiring multiple connections with the server, the client only has to download the unified css/js file once. However, this specific file is not re-created every time you make a change to your css or javascript. You will have to manually do it. Here is how:

In Drupal 7 go to:
Administration > Configuration > Development > Performance (admin/config/development/performance)  

and click the "Clear all caches" button. Notice that this will clear all caches of your site.

The method that I use more often though is to clear all the caches through Drush (if you do not know what Drush is, you should really check it out since it's a real life saver). Instead of navigating to the URL and clicking the button, you can just write the simple command:

drush cc all


in your command line and all caches will be cleared. 

Keep in mind, that if you are still theming/developing your site, you should disable CSS/JS aggregation. If you don't, you are adding more workload to the server that is rebuilding the unified css/js files every time that you press the "Clear All Caches" button. The specific option should only be turned on on production sites where css/js files have been finalized.