With the Groove Knowledge Base 2.0 we do not currently support custom HTML and CSS. The reasoning behind this is that we have built optimized, customizable themes which should work well for most businesses.
During the Groove 2.0 beta, we will be tracking requests and evaluating the need of allowing users to build their own themes in the near future.
If you'd like to cast your vote for this feature, please send a quick note to firstname.lastname@example.org and we'll add you to the list. :)