Using body the appearance of the canvas is being defined. Here and example of how it works:
Tab - Body
Sub Category

Round Corner Round Corner 7
Border Border Color #f20c0c
  Border width 2
Shadow Shadow On
  Shadow color #6e616e
Form Background Color    
  Color 1 #000000
  Color 2 #4a424a
  Opacity 1
Fonts Font Color #faedfa
  Fonts Settings What ever

And here the result:

Form Body Before
Form Body  After