This article is based on http://www.webmastersite.net/forums/index.php?sho...

Set your category selector options line (settings->general appearance) to this:

<option class="selectcolor{CATINDENT}" value="{CATID}">{CATINDENT} {CATNAME}</option>

Then in your stylesheet(s) add this:

option.selectcolor { background-color: green }
option.selectcolor- { background-color: blue }
option.selectcolor-- { background-color: red }

Plus continue adding lines for as many levels of indentation as you have.

Note: If you use a different indentation character than the hyphen, you will have to replace the hyphens with your character in the stylesheet.