There is a popular extension called "Peacock". It is useful for identifying editor instances when you have multiples of them open. This extension is included in the VS Code Can Do That extension pack.
Open the Command Palette (Cmd/Ctrl + Shift + P)
Select "Peacock: Enter a color"
Enter in any color
I find that it's most useful to set just the titlebar color with Peacock. This is enough to let me know at a quick glance which editor instance I'm looking at. For that reason, I disable the "Status Bar" and "Action Bar" theming in Peacock.