Mozilla Firefox is the browser I use for 99% of my online activities. On its best day it's much more secure than its competitors, but the big reason I choose Firefox is the limitless customization. Firefox is open source, so a user community has grown that develops its own custom themes, skins, colors, and add-ons, which are then offered for free to Firefox users.
One small drawback to custom user themes is that when Firefox is updated, like recently to version 3.5, the theme I was using is no longer compatible with the updated browser.
Theme creators are usually not far behind in getting their add-ons up to speed, but if you need more than the default grey browser until then, install the AnyColor add-on.
With Anycolor, you can choose from several pre-set color themes that are usually always compatible with any version, and the "Appearance" tab in AnyColor allows you to custom set colors with a palette control, or even use public streams from Flickr as your theme.
To get AnyColor, open Firefox and click TOOLS-->ADD-ONS, then type AnyColor in the add-on search bar.