Actually I much love this idea. I've been /msg'ing the tip to change their theme to monks who made such a mistake, and having it being done automatically for them seems like a good idea.

I do agree with hossman that the red theme would be confusing. Furthermore, I think the registration confirmation page or another one tied to the registration process should explain to users that the site is going to look different now, and explain to them why.

I think newcomers will gratefully welcome this feature. ++!

Makeshifts last the longest.