We shall refer to the mapping of Theme on to Subject as the UNMARKED THEME of a declarative clause. The Subject is the element that is chosen as Theme unless there is good reason for choosing something else. Note that this adds a further explanation for the use of a thematic equative in clauses such as 'you’re the one I blame, that’s what I meant': here the Theme is Subject, and therefore unmarked, whereas in the non-identifying form 'you I blame, that I meant', making 'you' and 'that' thematic also makes them marked Themes (because not Subject), and so adds a sense of contrast which may be out of place [...] A Theme that is something other than the Subject, in a declarative clause, we shall refer to as MARKED THEME. The most usual form of marked Theme is an adverbial group, e.g. 'today', 'suddenly', 'somewhat distractedly', or prepositional phrase, e.g. 'at night', 'in the corner', 'without much hope', functioning as ADJUNCT in the clause.
- Halliday (1985), a pag.45