It seems correct to put it in UIButton extension, but I am not sure that traitCollectionDidChange is a solution because you are not in your app when trait changement occurred and so your code is not executed.
Using sort of dynamic color seems to be a better option.
Define a light and dark color in your Color Set if you use Asset Catalog or use UIColor init(dynamicProvider:) initializer.
Something like this :
static var myBorderColor: UIColor {
if #available(iOS 13.0, *) {
return UIColor { (traits) -> UIColor in
// Return one of two colors depending on light or dark mode
return traits.userInterfaceStyle == .dark ?
UIColor(red: 0.5, green: 0.4, blue: 0.3, alpha: 1) :
UIColor(red: 0.3, green: 0.4, blue: 0.5, alpha: 1)
}
} else {
// iOS 12 and earlier, no dark mode
return UIColor(red: 0.3, green: 0.4, blue: 0.5, alpha: 1)
}
}
}
