Ok, if I understand how "Native" look and feel is supposed to work, controls you make will look OSX-like on OSX, OS 9-like on OS 9, and Windows-like on Windows. I can test that by making an app on Mac, and sure enough things seem to work, the buttons look Windows-like under Windows 8. I can't test the other way though, because although the OSX app I make under Windows is almost exactly the size of the one under OSX, it won't open. But, I suspect it would look right if it did open.
If Themes worked the same way it would almost be what you would want. But it needs to go a bit further, and have it be that the controls look iOS 5-like on iOS 5, iOS 6-like on iOS 6, and iOS 7-like on iOS 7. That last one will be the most noticeable one, and will be like a slap in the face to iOS 7 users if you have only put in support for iOS 6 looking controls.
Same with Android, the controls ought to look like they're from the version of Android that the device is using.
My fear is that Themes will only have generic iOS-like and generic Android-like. If it does have version specific looks then hopefully that will all be behind the scenes, and you won't have to use script to deduce which Theme should be used.
So, the best way things could work would be that apps don't know anything about how the OS needs to look, and that whatever you're looking at is a native control, that takes on whatever the OS want it to look like. For developing you would use a Theme to get a preview of how the control is going to appear. If need be there could be an interpreter layer that take mobile syntax and shows a desktop version of what the experience would be like.
The second best case is that controls are not native ones, but do at least look like the version of OS you're running on, and that the LiveCode user doesn't have to do any work to support the various OSes.