^ You have to collect 200 Mysterious albums, not Special Gifts (the ones you get from the castles)
I collected 200 albums and got the achievement. And then when you complete the Achievement by getting 5 stars you get the medal with an "A" logo.
I personally don't think the permanent castle skin will be available, but who knows maybe it'll be the reward for getting the 5th star.
Btw, to collect the Rewards for achievements you have to go to your profile, it won't notify you anywhere otherwise.