We have used css classes that SharePoint already implements for changing the look and feel of the forums. The reason for doing this is:

1, If you change the Theme your SharePoint site is using the forum will accordingly

2, If you implement your own classes based on the existing css names - again the forums will pickup the same look at feel.

The SharePoint css classes used in the forum are:

ms-descriptiontext,  ms-areaseparator, ms-standardheader, ms-toolbarContainer,  ms-pagetitle, ms-linksectionheader

Other classes are not specifically defined such as hyperlink styling - but this just gets inherited from the SharePoint css anyway.

One tool that is recommended is the IE Developer Toolbar. This will allow you to select elements of a html page and will let you know what css classes and styles are being used. You can also edit the style inline with the page so you can test and see how things will look.