Unfortunately this can’t be done. UIStackView is unusual in that it is a “non-rendering” view which performs layout (using Auto Layout constraints) but does not display itself. It has a layer like all UIViews, but it’s ignored.
See the Apple doc under “Managing the Stack View’s Appearance”:
The UIStackView is a nonrendering subclass of UIView. It does not
provide any user interface of its own. Instead, it just manages the
position and size of its arranged views. As a result, some properties
(like backgroundColor) have no affect on the stack view. Similarly,
you cannot override layerClass, drawRect:, or drawLayer:inContext: