I like that a lot of people are against themes cause that means that the few people that are brave enough to do one make it seem like a rarity that keeps it fresh.. I don't know how to word this, but like if every team in the world did a theme, it'd get boring.. but because only a select few do it every now and then, it's acceptable and sometimes actually really cute and well put together!
Our gym has done subtle themes, like one year every team danced to a throwback song, but the rest of the routine was still the average modern music.. We've never gone full out for a theme like some gyms have, and I personally wouldn't like it, but a mini/tiny/youth team would probably love it! You're right, it would get them into it more than just random top 40 hits should they still know the theme songs..