The complementary concepts "in theory" and "in practice" are expressed in Russian with different prepositions: в теории and на практике. Perhaps there is no good explanation of that, but if someone could provide any reason why the same preposition is not used in both phrases I would appreciate it.
На практике is probably modelled after на деле, and is a more abstract use of the на that is more or less grammaticalised as the "at" preposition for organised activities: на работе, на уборке урожая, на вечеринке, etc.
(Edit: or, now that I think of it, it may be something else: Russian may implicitly regard "unreal"/mental spaces as enclosed, and "real"/physical ones as open. Best illustrated by во сне vs. наяву.)
As for not using the same preposition — Russian essentially shrugs it off. Remember that the Russian multi-paradigm declension means that a speaker gets used to forming sequences of entirely different morphemes meaning exactly the same thing, e.g.
В лесу мы видели ежей, куниц[-Ø] и барсуков
— and that sense of equivalence in difference may carry over to prepositions. Speculatively, I even sometimes feel that в and на used with the prepositional case in a locative sense have come, subjectively, to be perceived as the front part of a circumfix, rather than a "detachable" preposition.
(That may also be a subtle yet powerful influence the на Украине/в Украине controversy, since for many people, using a different preposition would feel like an intervention into the deepest workings of grammar itself, rather than word choice.)
"na" literally translates to the english word on. like powering "on"
the letter B (pronounced VVV) means in.
so the Russians for reasons unknown to science have two ways of saying it. Both or correct you could say "в теории и в практике" (in theory and in practice) and they would both be correct for some reason. At this point it does not necessarily make a difference which word you choose people will understand you. however saying you are "на туалете" is strange but "на Украине" is correct... but "на России" is wrong..... so when talking about a location best use "v" (or in Russian alphabet B), whichever you choose V is your safe option.