Firebug is a free open source Firefox add-on that is an essential tool for anyone wanting to customize a WordPress theme. It allows you to find the exact place in the code that has to be changed for a particular issue. With firebug, you can identify the precise location of whatever you intend to change and it even allows you to do and view the changes in real time. Unfortunately, you cannot make a permanent change in Firebug but you can copy the code and go change it in your text editor like Programmers Notepad. Oh and be sure to add the addon to Firebug, called Firepicker which makes it easier to actually pick a hex color and see the change right there.
Lisa from How To Create A Website will show you more about using Firebug.
