What about a background image?
You could setup a set of different images that could be set ramdomly using CSS & a bit of PHP. All in the themes layout file.
https://css-tricks.com/snippets/php/randomize-background-image/
Cheers
Mary
You might think this is silly, but I added a custom user profile field to that allows an animated gif to appear next the user's name. It's sorta a 2nd profile pict. The main profile pict doesn't support animated gifs and I lock that part anyway so they can't change it for the rest of the year.
Students can choose from about 50 different animated gifs and they can change them whenever they want. For high school students, this is pretty cool.
So, there's no way really easy way to do this with CSS. Yeah I have to keep track of the code hacks, but I'm just a single high school teacher, not a university.
Here's the concept: