Align main content (more) to the right

Hopefully this question is in the right place now, for a child theme of twenty twelve.

Is it possible to have the content aligned (more) to the right – up to a limit? If the window becomes very wide, the left margin should get fixed, and the right margin increase instead. And if the window gets very narrow, the left margin should decrease before the content becomes narrower.

I’d like to keep the left margin between a minimum and maximum value (showing e.g. at least a 200px and at most a 400px wide section of my background image to the left of the content), but not at the expense of the content.

Or alternatively, the content could float freely, just aligned more to the right (e.g. margin twice as wide to the left than to the right).