It's not supposed to have a theme afaik.
Neither should it, imho. Monster Manuals should be adding varietly for every kind of campaign, i.e. all monster types over the complete level range.
Save themes for 'monster ecology' or 'environment' type books.
Too bad. I like the idea of themed monster books. It'd probably be bad for business, though: the typical monster book is somewhat like a random booster pack. You'll pick it up in spite of some of its contents.
I think the only theme I could see happening is to have one really important cover monster that has multiple-pages devoted to. For this MM it was Orcus, for next Demogorgon.
Now this may or may not be a actual theme, but it is the only hint of one I can see.