the crackberry store will start carrying themes once they start getting developed..As of now they dont have any..also people post themes in the forums so you can routinely do searches for tour themes
the 8900 has the same screen just using 4.6. i bet the theme builder will let you put the 8900 4.6 themes on the tour.
as far as i know.... for example.... you can take a 4.3 curve theme and put it on a 4.5 curve, but you cant take a 4.5 curve theme and put it on a 4.3 device.
with this you should be able to put 4.6 on 4.7 devices.