Das Thema der Ausstellung waren die Höhlenmenschen.

I am puzzled by the grammar in this sentence:

Hi sfpugh,

Your second theory is right: sein is here conjugated according to die Höhlenmenschen.

Lucia

That's interesting as in English I would never say "The subject of the exhibition were the cavemen".

I too am puzzled by this.  Far be it from me to question you, Lucia, but as a matter of interest I typed the English translation of the sentence in question into one of the online translators and it came up with the singular form of the verb.  It can be notoriously difficult in English to work out the subject of a sentence but I would have thought that here that the subject of the sentence is "das Thema" and so the verb would be singular.  Perhaps there are different rules in German that I am not aware of.

I spoke to a german friend about this her first thought was to to use waren, but then when she thought about it she agreed that  Thema  was the subject so it should technically be war, however many Germans are not particularly accurate with their grammar and might easily say waren.

 I wonder how it would be if the sentence was shortened to ​Das Thema war/waren die Höhlenmenschen.
I should think that in this case the subject is right next to the verb so war would be used?
How about it Lucia?

I did some further research on this in terms of the English parsing and concluded that under English rules of grammar, it was possible to argue that the subject of the sentence (which would determine the form of the verb) could be either the "theme" or the "cavemen".  Most people would just go with what sounds right and in English, the singular form of the verb sounds right.  It may be different for German speakers, which may be why sfpugh's friend's first thought was to use waren.  All very difficult!  On another note, I now wonder whether the online translators actually understand the rules of grammar or simply give a literal translation of an English sentence which is why the online translator I used came back with the singular war.

I am trying to remember my Latin parsing from many years ago. I think the verb to be doesn't take an object so that both sides of the verb are the subject. If you look at "Felix is a cat", both sides are the subject . But when one side is singular and the other side is plural, things get ambiguous.
 both sides are the subject - there is another one :-)
For me, in English, the left side is dominant  but as you say maybe in German it isn't.
May be Lucia can help?

On Google translate, I think it mostly translates literally.
The theme of the exhibition was cavemen. uses war
but The theme of the exhibition were cavemen. uses waren
 

At the risk of unduly prolonging this thread, I was still worried by the fact that I couldn't find a clear answer to sfpugh's question, so I did even more research and finally (I think) got to the bottom of the problem.
In the sentence in question, we have a subject (the theme), a linking verb (was/were) and a further noun as the subject complement (the cavemen).  If we take Fowler's Modern English Usage (rev. 3rd ed.) as the authority, it states that the rule to apply is as follows:  "When a subject and a complement of a different number are separated by the verb to be, the verb should agree with the number of the subject."
So the correct form of the sentence, in English, is 
"The theme of the exhibition was the cavemen."
However, my research also indicated that correctness is one thing but graceful English is another.  For example, people would tend to say "The thing that annoyed me the most were the grammatical errors", although strictly speaking the verb should be "was".  We get round this in English by rephrasing to say, for example, "What annoyed me the most were the grammatical errors."  This both sounds right and is also grammatically correct because in English, "what" can be either singular or plural.
Whether the same rule applies in German is up to you, Lucia, to say.
Overall, an interesting question which highlighted my ignorance of some basic rules of English grammar.  It reminded me of my schooldays in Australia long ago when I started to learn German.  At a very early stage in our classes, I remember our teacher, who happened to be Austrian, throwing up his hands in horror and saying in effect that it was impossible to teach us German when we did not understand the basic rules of English grammar!  So he proceeded to teach us in English the difference between dative, accusative and nominative cases and the like.  There were probably some interesting discussions in the teachers' common room between him and our English master!

 

It sounds as though you had a good teacher there.

I guess we have worked this to death, the rule seems to be pretty clear in English but not so much in German.
For what it is worth, I checked in http://www.duden.de/rechtschreibpruefung-online
and it accepted both war and waren, but rejected warst.  It commented that warst did not agree with das Thema so it is obviously doing some kind of grammar check. Maybe both are OK :-)