Please see FAQ entry . . . .
http://www.dew-code.com/content/view/20/25/
Customzing themes
Preview the included themes and find one that is closest to the look you want.
Copy that theme directory to a new directory, such as /themes/mytheme
The majority of the colors and overall look are defined the in the stylesheet, style.css
Depending on the theme you started with, there may be back ground colors defined in files like header.php and potentially a few graphics.
Using Firefox with the web developer plugin helps you find which class in the stylesheet to edit