Use OS color scheme to init light/dark mode (#2591)

[pt2121]

Closes #2591

I tried to implement what mentioned in the issue:

I think if you open up Dokka for the first time, it could choose light/dark mode based on OS theme, and then if the person decides to switch it to something else - remember that choice.

[IgnatBeresnev]

Hi! I think you're on the right track! Thanks for deciding to implement this

[pt2121]

I updated my PR.
If a user has never selected the Dokka theme, do you want to change the theme when the user changes the OS theme as well?
If so, I can add the listener and toggle the class.

[IgnatBeresnev]

If a user has never selected the Dokka theme, do you want to change the theme when the user changes the OS theme as well?

If you want, I don't think we'll object :) But I think it's fine the way it is - navigating to any page would trigger the script in the header, so the new theme should be picked up. We'll probably also have to re-do it anyway if/when Dokka migrates to react routing

[IgnatBeresnev]

LGTM, although I'm unable to test it properly (running a custom linux theme, so it would mess everything up). @vmishenev if you are able and have the theme toggle, could you please check?

Thanks once again for taking the initiative!

[vmishenev]

I have tested it via simulation in a debugger. It works if a theme switcher has not been used.