Hello Thanks for your reply.
In case of background color of long-look notification View, I recommend to use Custom SwiftUI VIew with WKUserNotificationHostingController than WKUserNotificationInterfaceController. I hope this could solve your problem!
diago_y
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Thank you for your reply.
I tried to find a solution but It seems like sashColor is not supported in 9.0 + OS.
In case of Notification View's background color, I reccomend to use custom SwiftUI view for notificationView and WKUserNotificationHostingController instead of WKUserNotificationInterfaceController.