Custom HTML5 background Storyline 360

Hi,

I am trying to find a way to insert a custom HTML5 background on my Storyline 360 courses. We were successful in changing the backgrounds in our Storyline 2 courses but have noticed that the 360 files have different coding. I have tried to follow tutorials for the new version, changing the css, but have been unsuccessful as the courses are still showing black backgrounds when uploaded. Below is what we changed:

cs-HTML {background-image: url("background.jpg") no-repeat 0 0; background-position: center;}

Any ideas?