Are secret service members civilian?

Yes - In the sense that ALL non-military law enforcement personnel are civilians, the Secret Service is a branch f the Treasury Department, they are not part of the military.

Added for clafification: Members of the law enforcement community will, among themselves, oftentimes make reference to "civilians." or "citizens," but what they are doing is differentiating between their status as 'sworn officers,' and members of the general public (i.e.: the 'non-law enforcement' community).