Hmmm, just starting delving into SharePoint 2007 and the first bit of feedback I got from one of my customers was that following the CSS classes (with somewhat obscure names) in SharePoint can be a challenge!!
Angus (and JohnH) pointed me to the "Internet Explorer Developer Toolbar" at http://www.microsoft.com/downloads/details.aspx?FamilyID=e59c3964-672d-4511-bb3e-2d5e1db91038&DisplayLang=en, thanks guys, very cool!!
Click here for an example of viewing a page with "View -> Classes and ID Information" enabled.
Also, Angus has a great set of getting started resources linked off his page – well worth a bit of time – see http://blogs.msdn.com/angus_logan/search.aspx?q=resources&p=1
Rob informs me that there is an equivalent for Firefox tooJ