I have the same problem Mohan37 hads some months ago:
'When I set the mode to Full Screen, the background image covers the page content and you can neither see or click on anything. It works fine with Normal mode.'
I use the Childishly Simple theme.
I've added this to the Custom Stylesheet box of the Image Set:
and it didn't work, plus I added '!important' after every number - also doesn't work. Is there anything else that can be done?