I like to scroll the body independently of the form background instead of scrolling it with the background. It currently does this in the Classic Designer when you are in the Themes tab and I used to be able to add a few lines of CSS to get it to do this. How do we do this in the Modern Designer? This CSS doesn't do it anymore.

body { 
  background-repeat: no-repeat; 
  background-position: center; 
  background-attachment: fixed; 
  background-size: cover; 
}

When the background scrolls with the form it requires designers to commission photographs specifically for this purpose. If it didn't scroll the background image we could use smaller images not created specifically for this purpose. 

Video to demonstrate:

https://www.screencast.com/t/UZJFMZ2wD