Since the normal wither already makes the sky a bit darker, i think this should be a thing here, too.
Especially in phase 4 and above where this huge thing is around.
In phase 7 it currently looks "boring" like this:

While it could look scary like this:

An optional (changeable in the config) thunderstorm at the very end of phase 7 would also be a nice idea.

The wither storm does not seem to darken the sky in phase 4.

Implemented as of the Prereleases