Style Sheet Inheritance - style sheet resources accessed from this tab use a different type of inheritance to the rest of the theme
Style Sheets 3.x - technical overview of the style sheets used in Theme Builder 3.x
Panel Classes and IDs - if you need to specify CSS that affects a specific panel, this lists the classes you can use to make your CSS more specific.
Style Sheets Overview- this shows how various CSS resources are included in the layout.
70 CSS tips and tricks - this external guide has loads of useful CSS tips and tricks but be warned that not all of them will work in a dynamic application like Confluence.