Halliday & Matthiessen (2014: 110):
By the same token, however, since these items are thematic by default, when one of them is present it does not take up the full thematic potential of the clause in which it occurs. What follows it will also have thematic status, almost if not quite as prominently as when nothing else precedes. We can demonstrate this by reference to the concept of ‘marked (topical) Theme’. On the one hand, after a continuative or a conjunction it is still possible to introduce a marked type of topical Theme, either in contrast or as a setting ... On the other hand, such marked Themes appear to be slightly less frequent when there is some inherently thematic item in the clause, suggesting that some of the ‘quantum of thematicity’ has already been taken up.