The Green Hornet was created by the same guy who brought us The Lone Ranger - Detroit radio mogul George Trendle. Both relied on public-domain classical music for their themes because Trendle was notoriously cheap and didn't want to pay for music rights.

Trendle also created Sgt. Preston of the Yukon.

https://en.wikipedia.org/wiki/The_Green_Hornet_(radio_series)