Best option is to use a custom skin. Set either a div tag with 100% width, Give it a background color you want to use for the "outline"  Inside that div tag create another one with width around um... 75%Setting the background color to white. From inside that tag put your content panes. The background color will fill the outer div tag which fills the screen, and the inner div tag will cover over that color with white. There will be a few other CSS tweaks needed to make sure all the content panes follow the colors they should, but its where I would go. The default skins are usually more hassle to alter, then it is to learn how to make lightweight custom skins.

If I get some free time tonight I will try to make up a base skin for you guys, any set limits you have to have? For example: has to fit 800x600, or will full work? Noting the middle won't be full.