It's only recently that modifying the css has become really popular with users. Even so there's quite a bit in the forum already.
This post gives information on how to determine what the css element names are for what you want to change.
This post has more info and a link to @devonzuegel 's userstyle.css file which is a great example to learn from and adapt to your needs.
Keep an eye on this thread as useful snippets appear such as @memophen 's tip for changing the search highlight and marker colours.
and if you ceate something in css that you feel may benefit others, please share it so people who later come to the forum can search for "css" and get lots of useful information.