I’d like to change the background color of designer screen from black to white.
Is it possible?
I mean, this black screen.
Howdy @takuyatakahama and welcome to the community! 
Unfortunately there isn’t any functionality to swap the Designer UI from “dark mode”, but this is something that would be perfect for the Wishlist.
A quick search didn’t yield any ideas similar to yours, but you can always post a new suggestion for others to throw some support behind 