This page lets you manually override the OS dark mode setting.
But is imperfect, it's so much easier if you avoid JS altogether.
I have added an element below to demonstrate the problems you will
encounter if you try to bypass the OS setting. Well, you will have to
write a ton more CSS to tackle several elements on the screen.
Notice the color of the scrollbar below when you toggle dark mode
using the button.
The scrollbar styles don't change, you will have to write additional CSS
to tackle that. Same goes for the toggle button.
Well, you can try to find the lesser known CSS properties to solve that.
And the scrollbar-color CSS property exists,
but it's experimental and isn't supported across all
So you will either be unsuccessful or will have to find all the vendor
prefixes to style the scrollbar for all modern browsers.
So I hope you're convinced that, it's much easier to use the
meta tag to let the browser's default stylesheet prevent you from
working so hard.