Breakpoint Backgrounds Bookmarklet.
Since this is just a debugging tool on a local machine, even the simplest methods will get the job done. The most interesting thing is the enclosing
!important to avoid worrying about specificity. Here's the whole thing:
Just load this up inside the location field for a new bookmark, and run it whenever you want to add the colors. Feel free to change the breakpoints to something for your site. It's just CSS on the inside, so you might have to reset a few more colors or adjust the selectors for your site.
For more on bookmarklets, see Bookmarklets FTW The Case by Lydia Katsamberis from Cascadia JS 2014.