Typically, Basic Webstores fill the width of the browser. Many web designers prefer to create a more narrow look. There is a quick workaround you can use to control the width of the Basic Webstore.

1. Go to Webstore > Basic Webstore > Design Center > Customize Theme > Header (Logo Area) and click on Advanced HTML Mode

2. At the bottom of the HTML box, add the following HTML tag:
<table width=?750? align=?center?>

(Please note: To be strict about your HTML, you should add a </table> into the footer somewhere.)