You can't change the dark workspace in X4 at the moment, but that option should be re-instated when the service pack is released.
There won't be any mention of this in the Corel User to User Web Board (who thinks up these names..).
There is a separate (closed) board for beta testing where it has been mentioned.
There is no date for the release of the service pack.
I'd rather wait a little longer and get the major issues (workspace colour, which is darker than X3, grey+black icons, which are awful), corrected.
A positive note: X4 is a lot faster than my ususal X2, can't comment on X3, I uninstalled it after trying it for 2 weeks.

Grenou