The Caspian and Modena themes are special-cased in various ways in JavaFX. For example, depending on the operating system or the ConditionalFeature set, auxiliary stylesheets are added to the user-agent stylesheet. Another example is the high-contrast mode on Windows, where the localized theme name is queried to replicate forced system colors with additional stylesheets.

We should remove all of that, and instead use new APIs like media queries and CSS system colors to implement the built-in themes. If successful, the built-in themes are no longer special citizens in JavaFX.