respect org.freedesktop.appearance dark/light theme setting

[RushingAlien]

Is your feature request related to a problem? Please describe.
Currently, the modern Linux standard for setting a dark theme, is the org.freedesktop.appearance key, which is a toolkit agnostic setting. I don't know if this is implemented in Flutter, but Soundux doesn't seem to follow it while rhymdport, another flutter app, does.

Describe the solution you'd like
For soundux to listen the the org.freedesktop.appearance key to set a dark theme if the key is available even when set to "no preferences"

Describe alternatives you've considered
Setting a dark GTK theme

Additional context

Desktop (please complete the following information):

• Environment: Linux Flathub

[Curve]

I don't know if this is implemented in Flutter

Soundux doesn't use Flutter ^^ But we do have an implementation for respecting the system theme, maybe it's broken somehow

[RushingAlien]

Soundux doesn't use Flutter

Ah, my bad. Just read that it uses webkit2gtk on Linux

But we do have an implementation for respecting the system theme

Don't worry, it works, but it listens to gtk-application-prefer-dark-theme instead of the new standard, org.freedesktop.appearance