Hi there , I know you said you know the trick about darkmode , But there is another trick where the webpage turns dark depending on the system scheme , it adds checkbox to theme option . The flag is “Darken Websites checkbox in themes settings”. Go to theme and activate the checkbox. Am sorry if this doesn’t work out for you.
The problem is that as now, even if you had the button on the main page, the browser needs to restart/relaunch to apply the flag, no matter where the button is, be it flags or a checkbox in settings