Officially, this course is no longer in beta. True, the tips still needs completing, but officially Duolingo recognizes it as phase 3 course.
As for the question itself: from my experiences in Spanish course it does seem like words are marked as orange even if it is another form of already learned word. So even though you might still recognize it, Duolingo will treat it as new words just in case.
The preposition "o" needs to have the adjective and noun following it to take the locative case in order for "o" to mean about. All plural locative nouns end in "ach", and all plural locative adjectives end in "ych", regardless of gender. In short: "o + locative" means "about". I compiled a list of Polish prepositions and the cases they take here: http://frogfaceengage.tumblr.com/post/148204575665/polish-prepositions
Here! I know it's not as good, but I've redone it as best as I can, hope it helps ^_^ https://hygge-in-fall.tumblr.com/post/167907955351/polish-prepositions-cases
Cases. When there's "o" in the meaning of "about", that needs the noun phrase Y ('about Y') to change into Locative.
First of all, jabłka were "duże" at the beginning. There are two plurals, masculine personal and not masculine-personal, and as apples are not a group of people with at least one man, they're not-masculine personal and therefore "duże" and not "duzi".
And the rest is just changing Nominative "duże jabłka" to Locative "dużych jabłkach".
Well, the second sentence maybe isn't ungrammatical, but it's strange. If you use "how", then you should probably go with "podobać się" for "like". "Jak ci się podobają TE jabłka?!" - 'podobać się' is more about first impression, or about the general... attractiveness of something. "lubić" is more general.
Anyway, no, I don't think this idiom makes any sense in Polish ;)
This is an interesting sentence... especially if your name is Newton !