Avoid the 'white flash' when using a dark theme

[frog44]

Users with a dark theme get a 'white flash' when starting Firefox. Obviously the initial default theme before the dark theme takes effect. Can the default start screen be dark (and change to light if necessary)?

[Torrocks]

I just spent 10 hours t/s why my theme was flashing purple incognito, and white in normal.  It was firefox the whole time.  I have to uninstall this now.  Can't believe this is even  a thing.