So I basically use only terminal apps (black!), a couple of messengers (dark mode) and Firefox. And yes, the problem is the latter. For a couple of years I used the Dark Reader extension. It works, pages look great - BUT! nothing would solve the occasional white flash problem. Last time I checked, it’s basically unsolvable by addons. Then I discovered the simple solution: Preferences > Manage colors and override the default colors! It works and it’s native! Pages sometimes look a bit ugly but always readable and zero white flash. This is a pro tip that hardly anyone talks about, you saw it here.