bbPress.org

There is a more advanced way. Edit your style.css file in your css editor, then upload to your site. Make sure you have a backup of your working stylesheet before making any changes.

For css editor suggestions:
http://www.hongkiat.com/blog/top-css-editors-reviewed/

I actually use Bluefish which is a combination css/html editor.
http://bluefish.openoffice.nl