Yeah, the style and alterations you've done are very good. Repeating what the others have said, the flash screen is a definite turn off, I would personally remove this completely due to it being very annoying waiting to get to the forums. Other than that nice work.
Edit: just noticed the xenforo copyright text is very hard to read and clashes with the pictured background. You may want to do something about that on the chance that it violates the xenforo TOS
I agree with all of the above. The only thing I really have to add, using my mouse to scroll from the top to bottom of the forum is pretty slow.
I'm not sure what would be slowing it down, but it definitely lags.