The particular problem is due to the minify plugin which is not maintaining the order of the stylesheets when minifying. Probably there is some setting in this plugin to prevent this from happening. Anyway, if you have no other styling issues, you can ignore this issue for now and fix the background issue by following steps –
1) Remove all the code there is in the file
2) Now to set an image background for all pages/posts (except custom home pages which are assumed to have sliders at top), go to Theme Options->Backgrounds tab and set the following option –
https://docs.google.com/file/d/0B9j6FQo4cSQFZDVSYjFmeE9sLUU/edit?usp=drivesdk
This will override the default but leave any custom header you have set on a individual post/page basis intact (as in the about us page in demo site). Thanks
3) If you need to set custom heading for a particular post/page (as you see in the about us page in demo/preview site), you can do so by go to options as seen below in the page/post/portfolio edit window –
https://docs.google.com/file/d/0B9j6FQo4cSQFX21HRmh6cXZ5Z28/edit?usp=drivesdk
Hope this helps.