I'm with you noeyedeer. It is a beast and there are no definitons for the word once you include the two letters, which I think is unfair.
I'm about half way but struggling over every clue. I took the precaution of writing down all the possible two letters from the theme, but it doesn't help much. I can't get 12d despite the hints. I have the same letters as Rocky7 and haven't managed to work out where the theme letters go.
But I can explain 1ac. It is the 5 letter word you are thinking of but the sides ar from left ro right and the theme letters are in the middle. I've been expecting them at the starts or the ends.
Similarly, I haven't got 49d, but I have got 50d and 75ac. I see you have the latter now. 50d definition is musical style. The clue splits Way (4), service (2) and brighten up (4).
Best of luck