Google acquired a company (a couple of years ago I think) that had developed a Unique (and patented) algorithm for theming internet content. This algorithm expanded the meaning of content to categorise websites using a controlled database of word meanings, roots and groupings. It therefore is able to place a page describing a Dining Table into the same themed category as a Chair. Thus a Site containing such pages may be themed as "Furniture".
I have read this Patent and can see it's possible/probable uses within the Search Industry.
Patenting an idea and implementing it for broad search across the Web are two very distinct and separate actions.
Other SEs are playing catch-up, but Google has a big lead due to it's Acquisition policy.
We all have theories and ideas and guesses. I cannot speak for anyone else, but I'll be glad to look at any Google queries you can think of where you believe theming is part of the solution.
But I look at queries all day long, have done so for years, and I've never seen any evidence that Google has implemented such an algorithm.
Your mileage may vary.