Simple Material Design Navigation Drawer BACKDROP 

(using CSS ::before)

Add this custom code CSS snippet to your webflow project:
(Project Settings -> Custom Code)


<style>
.w-nav-overlay::before {    

content: "";    
background: rgba(0,0,0,0.65);    
position: fixed;    
top: 0;    
left:0;    
bottom: 0;    
right: 0;  
}
</style>

Publish your website and see the magic happen :)