Actually, it enhances a lot more than background colors:
• all item-related displays, including inventory and popup-window item descriptions
• "You gain an effect:" messages from food, buffs, and other status modifiers
• the Combat Action Bar
• your character pane - a new look for sub-stat progress bars, new player quest-nudge, etc.
• headers for Familiar, Effects, and Last Adventure (which also has a hover-effect to remind you that it's a link)
• right-click menus - item options, chat pane shortcuts, and the Familiar Picker
• ALL of the bright blue (#0000FF) elements have a gradient, to make KoL easier to look at.
• similarly, everything that was square and boxy now has nice rounded corners.

Fun facts:
> This code is messy...and not completely finished. Help if you want.
> I add to it as I play KoL, and any parts of the code I've missed will probably be added when I get to it.
> You MAY get errors when you preview it in Stylish for Firefox. That's because of the CSS elements that will work in Google Chrome, but that Firefox doesn't yet recognize. Also because I'm constantly fixing errors and then adding more. (More code, I mean. Not more errors.)

02-08-2011: Added in the new Account Options. Mostly. :D
05-04-2011: Finally added in stuff for Alice's Army.
06-22-2011: Modified the part for the headers; added the gray and green headers.
06-29-2011: removed a misplaced semicolon; fixed Item Description margins
02-14-2012: did a THOROUGH re-haul of the code. (Man, it was ugly.) Everything should still work.
Firefox users, replace all -webkit- with -moz-.