Detect system theme in UI

[SuchithSridhar]

Is your feature request related to a problem? Please describe.
Right now the Headlight button just changes themes between dark and light - it would be nice to have an option to detect dark theme using the prefers-color-scheme option in browsers. This is especially useful in apps like Ferdium where you can't install the dark-reader extension.

Describe the solution you'd like
Provide the user an option to use system theme and then detect the state of prefers-color-scheme. In my personal opinion, this should be the default option as well.

Describe alternatives you've considered
One can use dark-reader but that's a rather clunky work-around.

Additional context
None.

[jmattheis]

I thought dark is our default. But yeah, I've nothing against using the preferred color scheme for selecting the default scheme in the gotify ui.