NationBuilder's robust theming system allows you to change the way your entire website looks with the click of a button. We have several beautiful official themes (each with style variations) which allow you to have a great looking website in seconds -- without touching HTML or CSS.
If you're comfortable with a little HTML and CSS, you can also create your own custom theme or make changes to official themes. To help you get started, we've added a custom themes screencast which covers the basics of how to do this. The screencast also explains the difference between theme and page level editing and introduces the liquid templating language and SASS, which we use to extend HTML and CSS.
We'll have an advanced theming screencast out before too long, but in the meantime, don't miss the theming documentation and object reference guide in the right column of the custom theme template page in the control panel for more advanced help.