How to enable such a check for all users and make it default?
I think the easiest way would be to just make your dark colour pallette the only available one, and then that would be your site colours.
But the aim is to match the user’s color-scheme mode of his device, so the light mode is needed too.
What are the possible ways to achieve that, API etc. ?
Ah, in that case would the
default dark mode color scheme id be what you’re looking for?
(The 'Enable automatic dark mode colour scheme` is default enabled, as long as there are no other colour palettes available to be ‘selected by user’)