Inserting html background into Superhero theme

The background I am loading changes in different formats (Monitor, iPad, mobile, etc.) I want to program the css with this

div {
background-image: url(plasma.png);
background-repeat: no-repeat;
background-size: 100% 100%;
background-origin: content-box }

#1. Where do I put this in the coding (and can I)?

#2. Does it belong in the Superhero theme that I am using or someplace else?

#3. Do I just replace “url” with the html of the image I want in the background?

Any thoughts or better ways of doing this is greatly appreciated. Thanks in advance!

The blog I need help with is justadad247.com.