Basicly, here is the line to edit:
PHP Code:
.content{background: url('../images/mainbg.png') repeat-y center; color:#000; margin:0 auto; padding:0; width:1019px}
You can modify the width:1019px to width:100% ...
BUT, (there allways is a but...) the design is based on some pictures too, and it is better to modify those, too. In WDFW theme I used a psd image made by me in photoshop, with the width of 1019 pixels ...