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.
Blogger Comment:
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).