random theme does exactly what it says on the tin, activates a random theme for new sites or blogs on your Multisite network.
So, rather than seeing the same old default theme on every new site, users get a much fresher experience.
Installation is simple, just activate the plugin from the WPMU DEV Dashboard and then network enable the themes you want to use.
After that new users who create sites will be allocated a random theme.
And, if you choose to use a resource like Farms 133 theme pack, then there are a heap to choose from.
Making your Multisite install that much more interesting.