You should first remove the fixed positioning of the menu (and any absolute positioned elements. Then apply your css (which is correct btw) to the body of the page, not the container class since there's actually 2 of those class in a clean indexhibit install (the menu and the content div).
The best would be to add an extra div after the body that would include the menu and content. (of course, you still need to remove absolute and fixed positioning since it just removes those parts from the normal rendering flow of the page).
The with is set in em, so it will change depending on your body's font size... be sure that's what you want. For a fixed width, use pixel based units. (an em based layout would be a nice thing though)