skin.default no longer works as expected

In every release of Sakai prior to 2.9.0, the skin.default property has behaved in a simple and predictable way. As of 2.9, there is now logic in the portal tool which changes the value of this property depending on the values of other settings in a way that is difficult to understand and explain, leading to confusion and delays at several institutions when building and deploying a new skin. While the new behavior has been documented in some places, it remains very confusing compared to the previous obvious behavior that required no explanation.