Use Firefox’s Firebug to identify the divs you want to make change to(use the right hand section in firebug to edit the width of these div etc.), then take a back-up of you stylesheet.After which you can modify your stylesheet.css file.Save the file and view the webpage in the browser.
The current theme uses a background image of a fixed widhtxheight .You will have to replace this image by your desired image(same widhtxheight as previous bk-gnd image) for best results.
Hope this helped.
