A Study to Learn How Safe ACT-777991 is and How Well it Works in Adults With Non-segmental Vitiligo

The purpose of this clinical trial is to learn how well ACT-777991 works, how safe it is and how well it is tolerated by adults with non-segmental vitiligo. The main question this clinical trial aims to answer is: • Can ACT-777991 help return color to the skin of the face of adults with non-segmental vitiligo? Researchers will compare ACT-777991 to placebo (a look-alike inactive treatment that contains no medicine) to see if ACT-777991 works to treat non-segmental vitiligo. Trial participants will: * Take the trial intervention (either ACT-777991 or placebo) daily for 24 weeks. * Visit the clinic 7 times for check-up and tests.