Out-of-the-box, if you're not using a custom CSS file, here is what's generated from your style settings (from the control panel):
.SelectedTab .UnSelectedTab .AltGridRow .NormGridRow .ContentBox .ContentBoxTitle .ContentBoxTitleNoBackground .ContentBoxBody
Specific to Forums, these have no interface for modification yet, but they're active:
.ForumCategoryHeader.MessageAuthorLinkCell1.MessageSubjectCell1.MessageBodyCell1.MessageTopBottomCell1.MessageToolsCell1.MessageAuthorInfoCell1.TopicIconHeaderCell.TopicTitleHeaderCell.TopicReplyCountHeaderCell.TopicLastPosterHeaderCell.TopicViewCountHeaderCell.TopicLastPostHeaderCell.TopicTitleCell.TopicIconCell.TopicReplyCountCell .TopicLastPosterCell.TopicViewCountCell.TopicLastPostCell.ForumTitleCell.ForumIconCell.ForumDescriptionText.ForumPostCountCell.ForumIconHeaderCell.ForumTitleHeaderCell.ForumTopicCountHeaderCell.ForumPostCountHeaderCell.ForumLastPostInfoHeaderCell.ForumTopicCountCell.ForumLastPostInfoCellThere are various other tags that are written out to the style, standard HTML tags like TD, A, A:visited, BODY, and all that good stuff.