Add ability to select dark or light mode.

[davidhooey]

The newer versions of FileZilla client implement the dark mode feature. However the mode is forced on the user based on the macOS settings. It would be great to have settings such as System, Dark, Light to allow the user to override the OS settings. Where System would adhere to the OS settings, and Light/Dark would explicitly set the theme overriding the OS theme. If the user prefers a Light FileZilla and a dark macOS, this is not possible with the current implementation.