What is the ruling on designing websites and forums where most sections are permissible but contain sections that violate Islamic Sharia? And what are the controls in this case?

As long as you know that the website or forum contains sections that violate Islamic law and include forbidden content, it is not permissible to cooperate with its owners by executing the design for them. This is based on the Almighty's saying: "And do not cooperate in sin and aggression." This is because the un-Islamic sections are intended, and the website design serves them, which would constitute aiding and approving them.