While I fully understand and support the desire and the idea behind this, I do see some challenges.
Would this mean that MusicBee should be aware of sunrise and sunset moments for all timezones?
And then when for a user in a specific timezone, 'day' changes into 'night' at a specific moment, should MusicBee stop playing (if it is) and restart using another skin?
Or should it only check the 'time of day' when starting MusicBee, and ignore it when day changes into night?
For now, my suggestion would be to begin with having the option for a user to define two skins that can simply be toggled between.
So, a user can set a light skin to be 'skin 1', and set a dark skin to be 'skin 2'.
And have a shortcut to easily toggle between them.
Either by key combination or by using a toolbar icon. (e.g. ☯ )