Not a 100% complete answer but at least something I found out in the meantime: extension NSDatePicker {     open override var frame: CGRect {         didSet {             isBordered = false         }     } } works perfectly well for the DatePicker(). The same extension for NSPopUpButton leads to having the right color and removing the color from the arrows (4) - the whole picker shrinks though and is pressed into a very small frame. Now I need to get it back to the right size and will be totally happy - but this doesn't really work out yet...