If you're gonna find yourself customising the CSS stylesheets of your Sharepoint portal then you have to get one of these two browser plugins.
First off we have the IE Developer Toolbar for Internet explorer:
And then we have Firebug for Mozilla Firefox:
Firebug is a really really nice little tool and it allows you to do inline / live CSS customization by editing currently applied styles or adding new ones on the fly. This allows you to tweak and tweak and tweak until you get your layout looking exactly the way you want it.
IE dev toolbar is a little bit more rudimentary but it does have the nice feature of enabling you to track a specific style to its file and definition.