Beginner - How to make CSS changes?
Find out what theme you're using (visit /admin/build/themes)
Find the files in your Drupal installation for that theme (if it's a core theme it'll be under /themes, if contrib it'll likely be in /sites/all/themes)
Change what you need to.


If it's a core theme, don't edit it (you'll lose the changes when you next update Drupal core) - create a subtheme instead.

The files in /sites/default/files/css are the optimised files produced when CSS aggregation is turned on; for development you'll want to disable that (/admin/settings/performance), or you'll need to clear the css cache every time you make a change.