In the last three seasons, the show went through dramatic changes. The show's humor was toned down significantly, the animation became darker, the color of the sky in each episode was changed to a continuous, ominous dark-red sky (commonplace with newer action-oriented children's programming at the time), the theme song was changed, the introduction sequence added in clips from the first live-action film, and the show took on a darker, more action-oriented atmosphere. The Turtles' demeanor evolved into a more serious and determined one than in prior seasons, and they devoted most of their time to tracking down villains. The series' main antagonists—Shredder, Krang, Bebop, and Rocksteady—who had previously been depicted as dangerous but comically inept villains, were now portrayed as a more menacing, unified threat. Additionally, Krang was revealed to have seized power in Dimension X through numerous betrayals and widespread destruction, resulting in old enemies seeking vengeance. Many recurring characters and villains were written out of the show by this point, with more focus placed on the main cast, and the destruction of the Channel 6 building in season 8 led to April working freelance.