Halliday & Matthiessen (2014: 123-4):
… the conflation of Theme with New is a regular feature. The sense is of course contrastive, because of the exclusive equation … It is this mapping of New and Theme, in fact, that gives the predicated theme construction its special flavour. …Since tonic prominence is not marked in writing, the predication has the additional function in written English of directing the reader to interpret the information structure in the intended way.