Twentytwelve changes width on me

make sure to set the custom background color to anything else than white; for example #fffffe

Why Is this? Thanks.

Why Is this?

what happens is:
there is a conditional check for the custom background color in functions.php which adds a body_class in case of #white or transparent, which in turn removes the padding and box-shadow of the .site style with some formatting based on that body_class in the style.css

Out of curiosity, is this like an intentional option, if you want 960px?

Thanks for this!