(Dialog): redundant gradiant on body

[Roohn]

Problem description

There seems to be a redundant background gradiant on the dialog body. This is only noticable when you try to change the background color of the body.
The gradiant is only redundant if there is a DialogHeader above the DialogBody.

Steps to reproduce

1. Style the DialogWindow with a different background-color (not white).
2. Go into mobile view
   See the part that is still white in the screenshot:

Expected behaviour

Only have the background gradiant if there is no DialogHeader above the body.