If you're customising CSS, Firefox is the best browser for the job - especially with these nifty extensions...
The web browser is by far the most standards compliant web browser currently available and will greatly increase your productivity when it comes to customising style sheets.
The CSS viewer plugin allows you to mouse-over any element on a web page to see it's ID and classes (if applicable) as well as existing styles applied to the element.
The Colorzilla plugin displays a little colour picker in your status bar which, once clicked, allows you to
FireBug is one of the most powerful tools for examining web pages - amongst other things, it allows you to interactively inspect HTML and CSS elements of a web page.