I don't think anyone has enough insider info to know all the theme weeks, but they've mentioned a couple in the press. I don't think they stop doing themes after so many years. They usually don't announce the full schedule of guests either.
The only info on instruments is the judges make noises that sound like they will be able to play them during the finales (so Top 13 and beyond) but taking anything the judges say as gospel as to what will happen isn't always a wise choice.
Spoiler information about themes and stuff usually is in the General discussion section.