In 2.9, the previously straightforward skin.default property became confusing and unintuitive because of the bug in
SAK-23395. Investigation as to why things seemed to work so inconsistently SAK-23396 led to the discovery of the fact that nearly every tool was doing something skin-related incorrectly. I've created JIRAs for the most problematic aspects, but not for everything, linked below. The community should consider either revamping the way skinning works, providing clearer documentation on the appropriate ways to use skins, or performing more static analysis on newly introduced core code such as lessonbuilder which wins for most files with hardcoded references to the "default" (but no longer default) skin.