First, I highly recommend taking a look at the Tiki Themes site (http://themes.tiki.org). There's lots of good stuff there.
As for your specific questions, to determine which CSS style to modify, you can use a tool such as Firebug (for Firefox) or Developer Tools (for IE). This chapter in my "Tiki Essentials" book may help.
To determine which template (TPL) file to modify, you can use the Smarty template usage indicator on the Admin Look & Feel page. Again, "Tiki Essentials" has a pretty good writeup.