Need quick css/media query help

So working on this site http://www.rapidturtles.org for a friend - for his guild in an online game he plays. When viewing the site full screen on a computer/laptop he wants the site width around 1000px (instead of the 940px the theme is set too) without losing responsiveness for other screen sizes. I know this can be accomplished fairly easily with css and media queries, but I'm still very new with learning css and can't seem to figure out the solution on my own.