The only leg theory in relation to cricket...

Perhaps it is an obsolete term. Sir Donald mentioned it more than once in reference to body line bowling as practiced by a Mr Jardin (if memory serves). He seemed to think that leg theory was all right but that body line wasn't. Kind of like brushing back versus intentionally beaning in baseball?

Great Aunt Fanny didn't mention leg theory at all. She did say that the traditional best man's joke about bowling a maiden over stopped being funny before WWI and was outlawed by the M.C.C. in 1923(?) so she appears to have some historical credentials.