What is the ruling on designing clothes or decorations related to holidays of disbelievers, such as Valentine's Day, New Year's Day, Halloween, and the eclipse, in order to sell them on e-commerce websites?

As long as the questioner seeks only what is purely permissible (halal (definition)), he must refrain from participating in or assisting with invalid holidays. This includes designing shirts specifically for them, based on the Almighty’s saying: "And those who do not bear witness to falsehood, and if they pass by ill speech, they pass by with dignity." Selling to them what they use to celebrate their holiday, such as clothes and food, is considered assisting them.