
Only problem you can run into is the fact that what if someone does not want the mobile version, but the desktop version. The implementation suggested will always force certain devices to certain themes.
If you're not bothered with that you just make two themes. One desktop version and one mobile version. What Tatane probably means is the fact that on the mobile version you want to skip certain elements from your desktop version. What you want to skip will always be for a webmaster to decide. He would know what to skip and what not. So creating a general mobile theme would not be that easy, unless it is responsive. But that would kinda miss the whole point, since you don't need a specific mobile version of a theme if your desktop theme is responsive.
Hope it made any sense
