The current Theme Designer looks boring and amateurish to me, so I think the Design Center can replace this.
The color picker
Currently in the Theme Designer, you have to choose from some pre-selected colors, and you have to add a custom color by typing in either the color name or a hex code. Don't worry; with the Design Center, let those worries go away!
The background and header image selector
In the Theme Designer, you have to upload the header image for your wiki by yourself; however, let the Design Center take all these worries away! I've chosen a number of pre-picked header images (which might need to be resized for fitting) for wikis made with FANDOM: