Darklight for SMP (Beta) - (UPDATED 9/10/2012)

I’ve been using this theme along with Thumbgen and I really love it.

But time has passed and I feel like asking, is there a way I could change the background image (the one with the clouds)?

I don’t need anything else. Just to know if it is possible to do it and how.

A quick search didnt yield the results I would like, so any suggestion would be appreciated.