From Textual To Topical Theme

Halliday & Matthiessen (2014: 125-6):

However, we have also seen that there is a compensatory principle at work whereby, if what comes first is ‘fixed’ (in the sense that its being first is an essential or at least typical characteristic), then what comes next may retain some thematic flavour. If the initial element is there as the expression not of thematic choice but of some other option in the grammar, then what follows it is also part of the Theme. We have embodied this in a general principle of interpretation whereby the Theme of a clause extends up to the first element that has some representational function in the clause (the ‘topical’ Theme). Hence in a dependent clause such as if winter comes, one part of the Theme is the if, expressing the nature of the clause’s relation to some other clause in the neighbourhood, and the other part is winter, which has a function both in transitivity (as Actor) and in mood (as Subject).