macOS systray icon needs to detect theme more often

[micahflee]

macOS has light themes and dark themes, but starting with Catalina it also has an "auto" mode which switches between the two based on the time of day. GPG Sync chooses which systray icon to use based on the current theme (light or dark) when it first launches. But if you're using "auto", then some of the time it's using the wrong icon.

On a regular basis, GPG Sync should detect if the theme has changed and, if so, change systray icons. Otherwise it ends up looking like this: