Halliday & Matthiessen (2014: 125):
There is thematic structure, in fact, in all major clause types: that is, all clauses expressing mood and transitivity, whether independent or not. But, as we have seen, there is a kind of scale of thematic freedom: whereas in a free declarative clause the speaker has a free choice of Theme — other things being equal he will map it on to the Subject, but this is merely the unmarked option — the further one moves away from this most open-ended form of the clause, the more the thematic options are restricted by structural pressures from other parts of the grammar, pressures that are themselves thematic in origin. In interrogatives and imperatives, and even more strongly in clauses that are not independent, the thematic principle has determined what it is that will be the Theme of the clause, leaving only a highly marked alternative option (as in interrogative) or else no alternative at all.