Theta is short-hand for Theta-class lifeforms, essentially human clones created by noted Colonial scientist Dr. Ravashol, who was kidnapped by the Cylons and forced to work for them (TOS: "The Gun on Ice Planet Zero, Part I").
Ravashol is allowed to create the clones to aid in his work and are used as slave labor. The Cylons mandate that Thetas were to be sterile to maintain purity of the life-form, but this sterility would later correct itself, forcing the Thetas to hide their children in caverns of their underground city. Thetas revere Ravashol, giving him the title of "Father-Creator."
Cylons treat these clones with greater distaste than the human Colonials, considering them to be sub-human and inflict various mistreatments to Thetas. It is due to this, and the fact that they were able to procreate, that incites the Thetas to rise up in opposition to the Cylons. Despite initial reservations, they aid Captain Apollo's expedition in destroying the Ravashol pulsar and the Cylon garrison on the planet. Ravashol commits himself to the Thetas, claiming that he has other inventions and means to resist the Cylons when they return (TOS: "The Gun on Ice Planet Zero, Part II").
Roles
Each Theta is given a specific task, such as Planner or Worker: Planners do the mental labor, while Workers do the physical labor. A female Theta series (e.g. Tenna) exists, and while their function is unspecified, they appear to be tasked with general caretaking operations.
Thetas typically wear similar clothing. Workers, such as Ser 5-9 and Tenna, namely off-white color coveralls under grey-color overalls. A color stripe runs down the coverall's shoulder and outer arms, and down the center front of the overalls. There are three distinct types of colors that the majority of men and women wear: orange, red, and green.
Planners are distinct in that they wear a hooded shirt instead of the jacket, and feature blue color stripes. A primary Planner, leader of the Clone Council that administrates the Theta's society, features two rows of the blue color stripes on his hooded coverall (but a single stripe on his overall).
Notes
To help illustrate that the society was clone-like, many of the background performers were either given beards and hair pieces (in the case of the Sers) to emulate Denny Miller's appearance or given masks to emulate Britt Ekland. This is discernible given the high-definition remasters of the Original Series episodes, and would not have been easy to discern with the restricted resolution and technology available in the 1970s CE.