If you have advanced customizations enabled on your Litmos account you can take control of the CSS stylesheet by entering your own styles in the “Account>Theme” section of your account.

To use a custom background the css required would look like the following:

   body{
      background-image: url(http://media.litmos.com/02723_vanishingbridge_2560x1440.jpg) !important;
   }
   .admin-body{
      background: transparent;
   }
   #page-body{
      background: transparent;
      background-color: transparent !important;
   }

Note: Litmos does not host your background image so this example assumes you have linked to an image hosted elsewhere.