[super_context_menu] Doesn't detect dark theme when run in Flatpak

[Merrit]

This appears to be a known issue with Flutter for some time.

Sandboxed applications like in Flatpak typically don't have direct access to system resources on Linux. The Linux solution is to use DBus to communicate with the system.

I created a simple package to get the brightness setting when running in a sandbox environment, where you can see how this is achieved fairly easily. I don't know if such an implementation is acceptable for these extensions, as it requires the dbus package - but its an idea. :)