The former treasurer for the Mashantucket Pequots has pleaded guilty to a federal theft charge. Steven Thomas entered the plea Thursday in US District Court in New Haven. He was indicted in January on charges that he stole $700,000 from the Mashantucket Pequot Tribal Nation, which owns and operates the Foxwoods Resort Casino in southeastern Connecticut. The tribal council announced Wednesday that Thomas had resigned his seat on the council. Thomas is accused of stealing the money between 2005 and 2008, when he was assistant director of the tribe’s natural resources department.