Make dark/light mode configurable

[mkrnr]

Is your feature request related to a problem? Please describe.

Currently, Plover always follows the light/dark mode of the operating system. Users might want to configure it.

Describe the solution you'd like

Have a setting to chose from:

• follow system (default)
• light mode
• dark mode

Additional context

Suggested by user Red on our Discord.

[mkrnr]

Implemented this for Windows and macOS with #1786 but for Linux we'd need a different approach since styleHints are not implemented there. Could be done via a more general theming configuration. Not the highest priority though.