This is kind of fun.
Go to your personal dashboard (that is the page you get to after you log in or when you click on the Home tab). Click on “Edit Theme” in the Account section. Select your color (hue) in the outer ring of the color picker, and then set you brightness and saturation in the middle square. Once you have a color you like, click on the “Update” button.
Darker colors really look good. Have fun, and let us know what you think.