Change system tray indicator icon to Dark or Light depending on OS dark mode preference

[fnune]

Version of Variety
variety 0.8.12

Describe the bug
One can only choose Dark or Light mode for the icon. But my system automatically changes from dark to light mode on schedule, and the Variety icon looks bad half of the day:

If we had an Auto option that respects and listens to changes of the user's prefer-dark-mode preference, it could be used to have a system tray icon that always looks good.

To Reproduce

1. Set the Variety icon to light
2. Turn on system dark mode
3. It looks off

Additional context
I have provided a reference implementation in a draft PR here: #721. I have not tested whether it works yet beyond checking that the Gtk preference is respected on my system (Plasma 6).