In order to find out the default CSS styles of a theme element and what CSS class or id it is using you can use the awesome Firebug addon.
Firebug is an extension for Mozilla’s Firefox (and Firebug Lite for Chrome) and helps you to find and change CSS elements in your WordPress theme. It overlays the rendered code of your site in a split screen so you can see and change the HTML/CSS, while being able to instantly view the change.
Learn how to use Firebug
Since there are so many perfect tutorials about Firebug available we just refer you to these tutorials:
- Using Firebug to Customize CSS (Studiopress)
- Customize your WordPress Theme using Firebug (WPMU)
- Firebug Guide for Web designers (Six Revisions)
Firebug Video Tutorial
You can also watch a Firebug video tutorial serie on youtube.