FireBug is a very handy extension/add-on for Firefox that allows you to customize a website's html/css locally without effecting the website itself. It is a good way to familiarize yourself with website design and wordpress theme development/modification.
You can then copy the style changes you've made to add them to the website's stylesheet for all to view.
This tutorial should help you learn a little more about what's under the bonnet of a WordPress theme and how to make basic changes.
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