What is the ruling on working to design a program that allows viewing movies and TV series, when only designing the interface and colors, and is it permissible to leave a job for its sake?

If the program displays films and series that transgress the limits of Allah, then it is not permissible for you to work on its design, because it is an aid to sin. You must politely apologize to your employer, and if he insists, then leave him and look for another [job], unless you are in dire need of it, in which case it is permissible for you to remain in it to the extent of the need.