"a" vs "the"

Looks fine.

It's fine. I assume there will only be one theme for the next mixtape. If so the is correct. If you said a theme it sounds like there will be more than one, and the one you choose will just one among many.

Thanks for your detailed answer. Yes, poll offers several themes to choose but only one will be featured in the next mixtape.

Thank you, a lot of people come on, get the answer and that's the last you hear of them until they want to know something else.

Strictly speaking, the intended reader will not be able to "choose" the theme, only to place a vote. Better to say "You can help to choose the theme".

You can choose the ("a"?) theme for the next Mixtape by submitting your vote via Youtube Poll Card or leaving a comment under this video

but maybe desirable. The "the" might be appropriate when you make up a theme, "a" when you're given a listing to vote from of if you just don't want to use "the" twice