display website same under different resolutions?

Hi, thanks for reading!
I’m trying to find out why my website is being cropped on the left and on the right with different screen resolutions… I’m using the retro-fitted theme and under every resolution but 1600×1200, a part of the header is missing, what’s really annoying. Tried to display the header 100% instead of px, and various other things, but I think there must be something in the stylesheet that states otherwise. I’ve posted it here: http://pastebin.com/wLVxWDse
Can someone please help me finding it?