Halliday & Matthiessen (1999: 239):
The significant point is that the Theme of an English clause has to be nominal. Not that there cannot exist other kinds of Theme — adverbials, prepositional phrases or even verbs; but these construe Themes which are highly marked (verbal Themes in particular), embodying features of contrast which are not appropriate in these contexts. The only kind of grammatical entity that construes the message in precisely the way required, without special effects, is a nominal — which may be a nominal group or else a nominalised clause or clause complex.