Halliday & Matthiessen (2014: 104-5):
Thus the question which element of the clause is typically chosen as Theme depends on the choice of mood. The pattern can be summarised as shown in Table 3-2. When some other element comes first, it constitutes a ‘marked’ choice of Theme; such marked Themes usually either express some kind of setting for the clause or carry a feature of contrast. Note that in such instances the element that would have been the unmarked choice as Theme is now part of the Rheme.
Note that this means that when a clause has a marked Theme, it does not have an unmarked Theme as well. The misunderstanding that a clause can have both a marked and unmarked Theme — along with many other theoretical misunderstandings — can be traced to Martin (1992).