One could make an argument that theory exists as a counterpart to strategy and similar to represent thought rather than practical application. We also hear a lot about "theorycrafting".
Considering we repeatedly question why the latter tag exists, though... it seems like this one is equally unnecessary.
In most scenarios, the presence of theory is unnecessary both to the question itself and to the question's place on this site - you can pretty much ask the exact same question from a practical standpoint and get the same answers. This also extends to truly theoretical possibilities, as "Is X possible?" is only different from "How do I do X?" in terms of semantics and approach. The rest of these tend to be about either terminology or game mechanics... neither of which are theoretical in the first place. As it were, most of the time the kind of question doesn't need to be tagged - identify-this-game and game-rec are outliers, not standards.
So pretty much any scenario that we could highlight theory, there's not much need point it out as its own tag. Simply put, "what if" questions don't really have a need to be specially distinct from "How do I" or "What is this" questions.
For your sole example, I'd suggest using cross-platform.