Firstly, this looks very similar to the way in which it has worked for ever without any warning or explanation. It needs more help, or at least a clue.

Issue 1:
The initial page with the different devices and their default theme is just meaningless. You don't get any clue as to what the page is doing (i.e. it is the first page of two). To make matters worse, it looks like the old page (that are probably expecting) with most of the themes missing and some stuff about 'devices' that you don't know what it means.

Issue 2: (scenario)

• On your desktop, pick the Mobile device (because you don't know what you are doing and don't know what this means).
• Great - you now get the full list of themes that you where expecting. There is NO mention anymore that you are changing the theme for mobile devices.
• You can now select any theme you like and nothing happens. Very confusing and counter-intuitive.

This desperately needs..
1. some text to explain the first screen because (a) it is non trivial in itself and (b) the functionality has changed from previous versions.
2. The 'Select theme' button is badly named. It selects the default theme for that device. This needs to be clear, 'Select default for device' is a bit long but at least you will have a clue
3. The second page (selecting the theme) needs (a) some big bold text saying what device you are selecting for and (b) a big bold warning that you will make absolutely no difference to the current theme if you are on a different device to the one you are changing.