Anytime I search for an addon via the search box in settings > add-ons manager I get all these theme results. Here is a search for "syntax" (via the add-ons manager) I had to make it very zoomed-out to fit long page into screen cap:
I use themes personally to visually differentiate between profiles. And I have nothing against fun and frivolous user customizations. Am not hating on the concept.
I am curious about why they are so aggressively pushed so that they show up be default when trying to search for add ons you need to toggle off every time. Searching for an add-on to do something and searching for a theme that has some keyword included seem to me like totally different tasks and mixing them up is a strange choice.
Is this like a major things firefox thinks people like about it? Do people like it?
Look at the user counts. This is exactly what it is. People like it enough to download it.
As for why mixing them up... Well... That's just classic store search things. "You may like this too" or "You may be in the wrong place to browse themes"