MyAnimeList (MAL) lets the user upload CSS to customize their user profile, for example, my profile is here. This can be done by going to your list, clicking the gear icon and "Style Settings". From there, clicking on a theme brings you to page where you can add custom CSS as well as upload a cover or background image. Further settings can be found by clicking on "List Settings" under the gear, which allows you to do things like turn off images and choose which columns are shown.
Notable users: