Hi there,
This might be a little tricky.
First, you would need to add an element into the “Before Header” hook in GP Hooks.
<div class="background-image"></div>
Then you would add this CSS:
.background-image {
position: absolute;
left: 0;
top: 0;
background-image: url( 'URL TO YOUR IMAGE' );
width: 100%;
height: 100%;
-webkit-filter: blur(5px);
-moz-filter: blur(5px);
-o-filter: blur(5px);
-ms-filter: blur(5px);
filter: blur(5px);
}
Most likely need to be tweaked as this is untested, but should be a solid start.
You can update GeneratePress in “Appearance > Themes”.