I may be wrong but I made this little change in stylesheet.css
and wrote 1600 instead of 990,
so before it was
/* Boxes layout */
body.custom-background #body-core {
max-width: 990px;
display: block;
margin: 0 auto;
and now
/* Boxes layout */
body.custom-background #body-core {
max-width: 1600px;
display: block;
margin: 0 auto;