Dark theme support

[miguelbaldi]

Is your feature request related to a problem? Please describe.
Let user chose if he wants dark or light theme.
Ideally could detected system preference and set theme accordingly.

Describe the solution you'd like
Could try to detected system preference.
Offer an application system configuration to override system behaviour.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
See: StyleManager
See: rust-dark-light
See: libadwaita styles and appearence