Respect OS color scheme

[twihno]

Currently, the client UI defaults to the dark color-scheme and has the option to switch to the light variant via a button.

This isn't a good default because

1. The user already has a preference (prefers-color-scheme) which isn't respected
2. Automatic theme switching doesn't work (e.g. on Android/iOS/some Linux systems)
3. The change via the button is still static and doesn't use the media query

This change updates the color change functionality to default to the browser provided values instead.

It also updates the logic of the theme toggle. If the new theme matches the preferred theme, the override is removed and the theme is set to "auto" again.