many themes use a wrapper div to center the whole site.
if you want your header(backgeound) to stretch across the browser window, make sure the header div is outside (before) the wrapper div, and has a inner div to center the header title, etc.
schematic:
‘normal’:
div #wrapper
div #header
header content
/div #header
div #content
posts,sidebars,etc
/div #content
div #footer
footer content
/div #footer
/div #wrapper
with flexible header and footer:
div #header
div #header-inner
header content
/div #header-inner
/div #header
div #wrapper
div #content
posts,sidebars,etc
/div #content
/div #wrapper
div #footer
div #footer-inner
footer content
/div #footer-inner
/div #footer
hope that was what you wanted to know, and you get the idea