Backgrounds and borders are comprised of immutable and final types. The following types are marked with the `final` modifier:

* Background
* BackgroundFill
* BackgroundImage
* BackgroundSize
* Border
* BorderWidths

The following types are not marked with the `final` modifier:

* BackgroundPosition
* BorderImage
* BorderStroke
* CornerRadii

This is probably just an oversight, as there is no value in allowing a subsection of these types to be extended.