Hello,
Sorry, if I understand you correctly, you can use bodyCls of the LayoutConfig...
1. UniHTMLFrame -> LayoutConfig -> bodyCls = myframe
2 CustomCSS:
.myframe {
height: 100%;
margin: 0;
background-repeat: no-repeat;
background-attachment: fixed;
background-image: linear-gradient(#E2FFA3, #C5DE8E);
max-height: 100%;
top: 100px;
position: static;
font-family: "helvetica";
}