To implement a theme CSS, the best thing is to modify a CSS for an existing approved theme. The public styles are the following:

pagebody: to be inserted in the body tag

titlebig: big titles
titlestandard: standard titles
titlemini: small titles or subtitles

textstandard: normal text
textmini: smaller text
textform: form elements

tablemain: to be inserted in the table tags
tableheader: to be inserted in td tags holding a title
tablebody: to be inserted in td tags holding content

passed: text to notify a successful event
failed: text to notify a failed event

enabled:  text to convey that something is enabled
disabled: text to convey that something is disabled

picture: to be inserted in img tags