The gymnastics rules and guidelines are set by The Fédération Internationale de Gymnastique (FIG). They set rules for everything related to artistic gymnastics, rhythmic gymnastics and the other types of gymnastics.
The guidelines for the apparel used by gymnasts, usually a leotard, are different for artistic gymnastics and rhythmic gymnastics. In artistic gymnastics the designs are more conservative and no skirt is allowed but rhythmic gymnastics involves complex colorful designs that often include a short skirt, mesh type materials in the color of the body and thousands of crystal that cause the leotard to almost glow.
The following article explains in more details the FIG guidelines for rhythmic gymnastics leotards