Sometimes it takes a day or more to get back to everyone so I appreciate your patience.
First you need to finish the theme setup. You see the two bright red messages in your menu area:
Set Main Menu - Left in "Appearance > Menus > Theme Locations"
Set Main Menu - Right in "Appearance > Menus > Theme Locations"
Those are in bright red not because it’s stylish but because they’re really important. If you don’t finish those steps you’ll have errors on your site that can cause other problems.
Regarding the background color extending the width of the browser, you should use the skins that were included as examples of how this can be done. I provide them so you will have reference materials. For example, when you open the CSS file for Skin 3 you will notice it has a style element not found in the others:
#SubHeader.fullWidth { background: #99271F; }
That’s what you’re missing. You need to add that to your skin CSS file (or in the Custom CSS field of the default design settings in the admin) but with your background color:
#SubHeader.fullWidth { background: #3DB1FF; }