Yes but that's not a randomizer that alters anything at each login, it's just giving the users the ability to set their own look. Here's a site I used to purchase themes from using their club subscription a few years ago when I was doing Joomla sites. Has even got some animated color changing or pulsing type backgrounds, pretty slick. This is the demo page and if you check out the 2 themes below you'll see what I mean:
http://www.yootheme.com/demo/joomla
Don't forget to check some of the variations of each theme. Many come in different colors, designs, etc. for the same theme which you can switch around at the top on the demo pane.
Spark is the first one you see, that one has animated coloring
Motion has an animated header image as well as rotating image banner under it
The one with the most changing would be called Daylight (on page 2 of the list) which changes the theme look several times a day according to the time of day it is and adjusts the lighting of the images accordingly. Very slick.
Pulse is one of the first ones they did that altered things on an animated system which changes the colors brighter and darker, alternates the colors, etc.
Anyways that's what I think he's after, but I don't have any idea where you'd find scripts for it for HTML as these are all CMS (content management systems) that are like Wordpress types (only way better lol) so they are built with a lot of PHP in them.