Look at Colin's post above. The path to the image has the theme in it. Without looking, I suspect the values would either be "blue" or "gray" (or maybe "grey" depending on how they spell it).
I will suggest that you really give some thought to whether or not you actually need (and want) this. How often are you going to be changing themes? Is this a feature you're going to be giving to your users? If so, why? Is this such a critical feature to have?
Additionally, as Colin mentioned you've now made your application tightly coupled to internal portions of the GXT API. Portions that are not meant to be used by API developers. Just because you can do something, doesn't necessarily mean it's a good idea. And in this case, I would strongly advise you, as Colin did, to just make your own ClientBundle. There are literally thousands of free icon libraries out there. http://www.iconfinder.com is a good place to start and is where we got our icon set.