Denis “Denny” Rice, a former Tiburon mayor who also served on the Marin Board of Supervisors and mounted an unsuccessful campaign for U.S. Congress, died July 19 in Hampton, N.H., after a long illness. He was 90 years old. Mr. Rice’s civic career began when he was elected to the Town Council in 1968. He served one four-year term, taking two turns as mayor. In 1976, he was elected to represent Southern Marin on the Board of Supervisors, where he also served one four-year term. In 1992, Mr. Rice r