Before you start styling...
You can now use styles like YouTube Dark'n Red, Facebook - Dark Shiny Blue, transparency, and Hipster Fo Sho ▲ (with space background). Start browsing the site to find other styles.
After installing styles, you can turn them off by going to Tools, Add-ons, User Styles. For more info, see the help docs (but really, just start using it, you'll probably figure it out). If you need help, ask in the forum or send me an e-mail.