"Themes" control the looky-feely of this site. You may choose among the installed themes to select the look that you prefer.
For registered users, click on "My Account," then click the tab labeled "Edit" and it will let you set your account preferences. (Just leave the Password boxes empty unless you want to change your password.)
Toward the bottom of that screen, it lets you pick which "theme" it will display for you.
Click "Submit" when done.