My english isn’t the best but I will try to explain.
You go to your control panel.
Presentation > Edit
Open Stylesheet (style.css) code.
Search for something like:
“body {
font-family: Arial,Helvetica,sans-serif;
font-size: 13px;
color: #333333;
background: url(“LINK TO YOUR IMAGE/BACKGROUND”) no-repeat fixed center top;
line-height: 1.4;
}”
Hope it help.
https://www.zemtb.com