Halliday & Matthiessen (2014: 101):
In a WH- interrogative, which is a search for a missing piece of information, the element that functions as Theme is the element that requests this information, namely the WH- element. It is the WH- element that expresses the nature of the missing piece: who, what, when, how, etc. So in a WH- interrogative the WH- element is put first no matter what other function it has in the mood structure of the clause, whether Subject, Adjunct or Complement. The meaning is ‘I want you to tell me the person, thing, time, manner, etc.’.