Possible to change total width of theme?

Is it possible to change the total width of the theme from 747px to i.e. 900px? I like the overall look n' feel of the theme, but think its a bit to narrow...
I have tried to tweak the css-stylesheet, but keep ending up with a gap between the borders of header-section, and the borders of the rest...
Please see the attached screenshot.
Thanks for all help.