By a constituent with the pragmatic function Theme we understand a constituent which does not form part of the predication, but precedes it and creates, as it were, a kind of framework within which the predication is to be interpreted. Examples are:
(24) de Pompeio, et facio diligenter et faciam quod mones (`As for Pompey, I am doing my best and will do what you advise me to do', Cic. Q. fr. 3.1.9)
(25) de forma, ovem esse oportet corpore amplo (`As for the form, a sheep ought to have a large body', Var. R. 2.2.3)In these examples, the de-constituent does not play a role in the predication; this is not the case in the following examples.
(26) de hoc homine … sic scriptum accepimus … summam fuisse eius … temperantiam (`As for this man, we have learnt that it was written that his self-control was extreme', Cic. Tusc. 5.57)
(27) homines maritimi Syracusis … cum eius cruciatu atque supplicio pascere oculos … vellent, potestas aspiciendi nemini facta est (`The sailors at Syracuse, when they wanted to feast their eyes on his suffering and punishment, nobody (of them) was given the opportunity to watch', Cic. Ver. 5.65)
(28) sed urbana plebes, ea vero praeceps erat (`But the city populace, it was truly rash', Sal. Cat. 37.4)In (26), de hoc homine is taken up by eius. In (27), instead of the nominative homines maritimi a genitive would be required (with nemini, which occurs much later in the sentence). In (28), finally, ea takes up the immediately preceding urbana plebes. The grammars classify cases of this kind in various ways, e.g. as anacolouthon and nominativus pendens,  often suggesting that they represent something unusual. In reality, we are dealing with a common phenomenon, and not with something ungrammatical. [8a]
Pinkster, Harm (1942-) , Latin Syntax and Semantics [info], xii, 320 p.: ill.; 24 cm. [word count] [Pinkster].