The Theme is a function in the CLAUSE AS A MESSAGE. It is what the message is concerned with: the point of departure for what the speaker is going to say.
- Halliday (1985), a pag.36

As a general guide, the Theme can be identified as the element which comes in first position in the clause. We have already indicated that this is not how the category of Theme is 'defined'. The definition is functional, as it is with all the elements in this interpretation of grammatical structure. The Theme is one element in a particular structural configuration which, taken as a whole, organizes the clause as a message [...].
- Halliday (1985), a pag.39

The theme is not necessarily a NOMINAL GROUP [...] It may also be an ADVERBIAL GROUP or PREPOSITIONAL PHRASE [...] The Theme of a clause is frequently marked off in speech by beeig spoken on a separate intonation contour, or TONE GROUP as it is called.
- Halliday (1985), a pag.39-40