Theta is a reference to Theta-class lifeforms, human clones created by noted Colonial scientist Dr. Ravishol, who was kidnapped by the Cylons and forced to work for them (Gun on Ice Planet Zero).
Ravishol was allowed to create the clones to aid in his work. Each clone was given a specific task as Planner or Worker. Planners were highly intelligent with incredible mathematical skill. They were also sterile. Workers were fearless, attractive, and unflappable, but not very bright.
Cylons treated these clones with apparently greater distaste than the human Colonials, whose opinions varied on their right to exist or be treated humanely.
One of the clones was played by actress Britt Ekland.
Links
Detailed page on Thetas as defined in a TOS-based multiplayer pen-and-paper roleplaying game