After some Googling ( https://www.google.com/search?q=iframe+z-index&ie=utf-8&oe=utf-8 ), apparently the items with Z-Index need to have a relative, absolute, or fixed position.
yes after googling myself I found the same thing and applied it to my CSS but still didn’t do it for me..
https://jsfiddle.net/75gL60s7/1/#&togetherjs=j852w8uAsk
An iframe displays another web page. It’s not the way to change your background! Use css directly. Also WP has different areas in the window. Background to what?
You need to learn to use the web inspector of your browser to determine what css is controlling what on the page. It can be quite convoluted. Then you need to make a custom style sheet.
