So, Basically we change colours or use one of the 2-4 themes, and then buy the rest. Super annoying considering that this is the second time Sony is limiting something that the PS3 allowed (Music being the other one).
I am guessing there is actually no good reason for this, other than to force theme purchases.
I'd say a good reason for custom themes not being supported yet is because they haven't finished a theme maker.
Regards to the music complaints, pretty sure Sony isn't purposely limiting their own console. They just didn't focus on making PS4 a media center console like they did with PS3, which i'm personally fine with seeing that I have a computer, a smartphone and a hundred other devices that can play music, videos etc. This newly added USB music player is a sign that they aren't ignoring media features completely and are most likely adding more down the road.