Just looking at your current theme, the forums are stretched beyond you header to the right as well.
If you look just under this image:
... you will see the forums stretch beyond the confines of your header even on the default theme.
I'm not 110% sure but I'm guessing the reason for this is due to this image:
...being too long.
I'd try dropping that image down to around 400 pixels wide and see how that affects your themes.