You are looking to make changes for main navigation and main content area both. Snow theme width defined with qa-body-wrapper class also qa-main ( content area ) has defined width. So you may required to change both.
Assuming you want to set main area width to 1200px
.qa-body-wrapper change width: 1000px to width: 1200px ( or whatever you want )
and for content area
.qa-main change width: 710px to width: 910px ( depend on what you have set for qa-body-wrapper)