Hey Horst,

Not entirely sure which background you want to change but here's the css codes that you can apply:

1. Main background, http://take.ms/JnRwg

[gist]
body .master {
background: #000;
}
[/gist]

2. Wrapper background, http://take.ms/wmG5q

[gist]
body .master .container > .wrap {
background: #000;
}
[/gist]