UI: respect browser's light/dark mode setting

[elsirion]

The old UI uses the browser's default while the new one has an in-app trigger. That trigger's default should be set to the browser's/OS's default.

[bansalayush247]

Should I keep the toggle button and store the user’s preference in local storage only when they toggle it?

[elsirion]

Up to you, I like having the override, but if too much work happy to remove.