What is the part where it shows how many posts are in a forum, usually called in CSS, because I can't find it.
CSS is Cascading STYLE Sheets, the number of posts in a forum would be a PHP variable, then stored in a database. What are you trying to do? I might be able to better answer your question that way.
If it's vBulletin: Login with admin account Go to Style Manager Use drop down menu that says "All style options" and change to "Main CSS" It gives you a description of each, so you know what you're changing. Click the old color and it will let you pick a new one. Scroll down and save when you are done. If you're using PHPBB, Invision, or anything else, can't help ya sorry
As far as I can remember it can be changed within the Admin Panel. So assuming you know how to get into the Admin panel, well all you do is sign in as Admin, and the link is at the bottom of the page, you then go onto Styles or Presentation tab something like that. This then presents you with the current colour settings for the theme you are using, and background images. Each area of the page has a different name so youll have to play around with it to see what area matches which. but it's all there.