Osshaft_adrian theme comes with a sample background image.
Open osshaft_adrian/css/osshaft-style.css.
At line 10:
body{width:99%;margin-left:5px;color:#323232;font:12px/1 Verdana, Arial, Helvetica, sans-serif;
background: #f7f7f7 url(images/background.png) no-repeat center center fixed;}
Change #f7f7f7 to whatever color you want. Background.png is default image, change with your image.
Background image at: osshaft-adrian/css/images folder.
Suggest you take a look at osshaft_11. This theme has 11 layout choices built in with one being similar to osshaft_adrain. These can be changed in admin panel.
www.osshaft.orgTom