Hello,
as it is stated in the roadmap, the 0.5 release of RoundCube will have a 'skin redesign and cleanup'.
1) Is it still being planned?
2) If yes, how significant the changes would be?
I'm willing to make a custom skin for RoundCube, which will be based on the default one but differ a lot from it. However, I'm not sure, if it's a right time to implement it, if layout, containers and JavaScript's would change a lot.
Skin redesign and cleanup is made all the time, but we do not plan any revolutionary changes in this version ;) It's more likely that something will be changed before beta release than after.