Halliday & Matthiessen (2014: 99):
The ‘most marked’ type of Theme in a declarative clause is thus a Complement: for example this responsibility in this responsibility we accept wholly. This is a nominal element that, being nominal, has the potentiality of being Subject; which has not been selected as Subject; and which nevertheless has been made thematic. Since it could have been Subject, and therefore unmarked Theme, there must be very good reason for making it a thematic Complement – it is being explicitly foregrounded as the Theme of the clause.*
* It is also likely to be given the status of New information within its own unit of information. At the same time, some element other than the Complement will be a candidate for the status of New within the Rheme of the clause, as in this they should refuse.