Forum Changes For April Fools

For April Fool's day (yes, I'm planning that far in advance!) I want to prank all my members by radically changing the theme of the forum to something daft (different background/banner/music/icons etc). This is going to take a lot of time to implement though. Is there a way I can create an entirely new theme like this without it going live on the forum? So I can build it up beforehand then just activate it on the day.

I don't want to have to put the forum into maintenance mode to change everything around, so if I can do it secretly in the admin panel it would help a lot. Also, is there a way to save the old theme so I can just click and swap it back once the joke's over?

Thanks for your help!

You could use a test forum to build it up maybe or make a theme.