Keep in mind that the method outlined will work, as long as those themes are "tweaked" by the developers for each new release of MATE.
MATE (as well as all desktops) appear to be moving away from fully customizable "themes" (hence the reduction in the numbers of included theme families, and adoption of the newer Yaru as the new default), but seems to be moving towards what is referred to as Client-Side Customization (CSC).
On the theme side, "adwaita" seems to be "infecting" all themes with their XML/CSS approach to themes. CSC is trying to implement a similar XML/CSS driver to application interfaces.
Whether those two share a common code/library base is unknown to me.