Hi there,
try adding this CSS:
div.slideout-overlay {
left: 264px;
}
#generate-slideout-menu.main-navigation {
background-color: transparent;
}
#generate-slideout-menu:before {
content: '';
position: absolute;
top: 0;
right: 0;
bottom: 0;
left: 0;
background-color: rgba(255,255,255,0.85);
backdrop-filter: blur(3px);
-webkit-backdrop-filter: blur(3px);
}
It will make the background transparent, and add a little blur ( where supported ) to the content behind the off canvas.