Facebook has always been notoriously difficult to customize. Personally, I think this is an improvement over MySpace's totally open platform (some people should not use code), but users should still have the option to change a few things if they want.
While you'll never be able to choose your own background image or add an obnoxious number of aWeSoMe quiz results to your profile, there are a few browser plugins that let you at least change the color scheme.
Color My Facebook is an extension (for any web browser) that allows users to change the layout color from the trademark blue. Once you install the application, Color My Facebook will change the blue to pink, but you have the option to choose any color after that. If your friends have the same add-on installed and have changed the color, you will be able to see the color they have chosen, as well.
Out of all of the add-ons out there, this Chrome extension is the most popular. Color Changer for Facebook allows you to not only change the color of your Facebook, but customize the top Facebook bar, friend names, and the main headlines, feed texts, and background. It might be more work for some, but if optimal customization is your thing, then this extension might be what you're looking for.
iStyla, the creation of a 14-year-old web developer, offers users several themes and presets that can be found on the iStyla Store. Other users that have iStyla can see your themed profile and vice versa. The only thing missing with this extension is the ability to customize.
How do you customize your Facebook?