Conjugation of the linking verb "sein" with a subject and subject compliment

In lesson 19.4, we are presented with the sentence, "Das Thema der Ausstellung waren die Höhenmenschen."

I'm confused. I would have used "war" instead of "waren" based on the fact that the subject ("das Thema") is singular, even though the subject complement ("die Höhenmenschen") is plural. Is there an error in the above sentence or does the grammatical rule in German state that the linking verb "sein" is conjugated based on the subject complement and not the subject?  

Julia -

Thank you for the explanation in the thread. Interesting that sfpugh and I were both confused by the same sentence.

2 years ago when this thread appeared in the Forum, sfpugh pointed out that a short explanation in the lesson would have been helpful in avoiding the confusion. The response to him at the time was that his feedback would be forwarded to the Course Development Team.

For whatever reason that his suggestion was ultimately not implemented in the lesson, in light of fact that the same confusion has again arisen, I wish to reinforce sfpugh's suggestion for the sake of future German learners.
 
Regards, 
PaulS108