When designing, I sometimes use this as my Template.
Add the CSS code to Web Properties > Website > HTML Code (head):
<style>
html {
background: url(https://picsum.photos/2400/1800?blur.jpg);
height: 1280px;
background-repeat: no-repeat;
background-position: center;
background-size: cover;
}
</style>
Preview and cycle the pages.
Change the width & height numbers and drop the ?blur as you see fit.