Twentyeleven gives you the option to place the navigation side-bar left or right:

What if you wanted to use the twentytwelve theme, and have the navigation on the left? And just to be different, let’s have it do that only when the screen width is greater than 800px.

One way to do it would be with a style.css file in a child theme having code like this:

To add some additional modification, make a new template file to change the footer. The child theme directory looks like this:

That screenshot.png shows up in the Theme Details window: