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. Chrome and Safari have a built-in equivalent, which you can access by right clicking on any section of your site and selecting Inspect Element.
You can then copy the style changes you’ve made to add them to the theme custom.css file to set your custom styles.
This tutorial will help you learn how to use Firebug to explore your WordPress theme and how to make basic theme style changes.