Wikipedia page for GM Granda from Peru says:
Julio Granda does not study opening theory. He has said that he has only read a chess book, the third volume of the General Chess Treaty of Roberto Grau. Anyway, he admits reading magazines and reviewing the Informant, which together with his excellent memory and assimilation capacity, has allowed him to win important tournaments and titles.
Google translation from spanish wikipedia.
Is this possible? Can one become a master without studying theory at all?