Halliday & Matthiessen (2004: 73, 74):
In a declarative clause, the typical pattern is one in which Theme is conflated with Subject; … We shall refer to the mapping of Theme on to Subject as the unmarked Theme of a declarative clause. The Subject is the element that is chosen as Theme unless there is good reason for choosing something else.
A Theme that is something other than the Subject, in a declarative clause, we shall refer to as a marked Theme. The most usual form of marked Theme is an adverbial group … or prepositional phrase … functioning as Adjunct in the clause. Least likely to be thematic is a Complement, which is a nominal group that is not functioning as Subject — something that could have been a Subject but is not … . Sometimes even the Complement from within a prepositional phrase functions as Theme … .
Note that such a clause has either an unmarked Theme or a marked Theme, not both. Those who stupidly think that the Subject is always Theme would have to analyse the clause on your left is the main bedroom as:
on your left
the main bedroom
that is, with two distinct 'points of departure' and the 'body of the message' being simply is.