Halliday & Matthiessen (2014: 119-20):
There is a close semantic relationship between the system of INFORMATION and the system of THEME — between information structure and thematic structure. This is reflected in the unmarked relationship between the two. Other things being equal, one information unit is co-extensive with one (ranking) clause (‘unmarked tonality’); and, in that case, the ordering of Given ^ New (‘unmarked tonicity’) means that the Theme falls within the Given, while the New falls within the Rheme.