Editing Image Header

I’m trying to get the screen header to stretch across the screen but it’s just not cooperating with me. This is the picture of it in the WordPress themes section:
https://i0.wp.com/themes.svn.www.ads-software.com/photoshoot/1.1.7/screenshot.png

So you would think that by default, the header would stretch across the screen like that but it doesn’t. Please help.

Thanks.