Thanks for the translations!

Would it be OK to modify footer.html within this PR so that the word "theme" gets added to messages.pot?

That sounds like a reasonable request.

Overall LGTM 👍 Some opinionated comments below, you don't have to implement them ✌️

msgid "Page not found"
msgstr "Strony nie znaleziono"

"Nie znaleziono strony" would be nicer to read.

#: mkdocs/themes/mkdocs/base.html:126
msgid "Previous"
msgstr "Wstecz"

If this is the previous/next page button, you can use the translation for that "Poprzednia strona" etc., as it would be nicer to read.
This doesn't apply to ReadTheDocs as "Previous" is used in other contexts than page switching.

msgstr "Dokumentacja zbudowana przy użyciu %(mkdocs_link)s."

Similar to the example shown in the OP, you could add flavour words to make it nicer to read / more grammatically correct e.g. "przy użyciu pakietu MkDocs"

msgstr "W tym miejscu możesz przeszukiwać dokumenty. Wpisz swoje kryteria wyszukiwania poniżej."
msgstr "Wpisz tutaj kryteria wyszukiwania"

Even though it's correct I feel like "frazę wyszukiwania" would be nicer to read, no need to keep it plural.

msgstr "Nie znaleziono żadnych wyników"

Unneccessary adjective "żadnych"

Thank you for the feedback!

That sounds like a reasonable request.

I added this in 6ff8e57. Hopefully this is OK.

Overall LGTM 👍 Some opinionated comments below, you don't have to implement them ✌️

Thanks. I tried to follow your comments, however I left the translations for "Previous" and "Next" in the MkDocs theme as is, because there wasn't much free space in the header.