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 ignore Halliday and 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.