As an example, consider the following written text, from a traumatic brain injured patient (from Prigatano et al. undated). Topical Themes are underlined [in bold], marked topical Themes are underlined in all clauses…
What would you like to do at the end of your rehabilitation program?
My first choice is to go back to work at the hospital.
This would be hard for me to do
because they have hired someone to take my place.
I liked the work.
I was there for over 5 years.
I also would like to go to a sheltered workshop.
This would be good,
but it has to have transportation to the place and home.
The transportation would have to be a bus
because I do not have a driver’s licence.
I was to have it renewed last year,
but that was not done.
To get a driver’s licence,
I will need to take the written and the driver’s exam.
The workshop will help me control my temper,
which is bad for me.
I get mad easily
and this is because of my brain injury.
All I can recall is that I was knocked out for 3 weeks.
I did my recovery at Mercy ICU.
I had my accident on my way to the hospital for a personnel director’s meeting.
The workshop might have been in Austin, Texas.
This means I will be away from home,
which I would miss very much.
I miss not being at home.
I would like to work in the yard
and work on the cars,
especially washing and wax(ing) them.
This I have not done for years.
The obvious candidate for macro-Theme in a writing exercise of this kind is the question: What would you like to do at the end of your rehabilitation?
 There are several points of evidence that demonstrate that this is not a writing exercise, nor even a text in written mode.
Firstly, it lacks the lexical density of written mode, and leans more towards the grammatical intricacy of spoken mode.
Secondly, it begins with an interviewer question addressing the patient 'you', and the rest of the text is the reply of the patient 'I'. The context of situation is clearly an interview with a patient in rehabilitation, not an exercise in a writing class.
Thirdly, it features a repair strategy that is common to written transcripts of spoken language: wax(ing).
The source of the text is:
Prigatano, G.P., J.R. Roueche & D.J. Fordyce, undated. "Nonaphasic Language Disturbances after Closed Head Injury". Presbyterian Hospital and Neurosurgery Section, University of Oklahoma. mimeo.
 This is not a marked topical Theme of a clause. It is a dependent clause in a 'regressive sequence' (b^a) clause nexus. It has thematic status within the clause nexus — not within a clause. See Halliday & Matthiessen (2004: 392-3).