Currently, the two themes shipped with JavaFX (Caspian and Modena) are implemented as a set of stylesheets and some internal logic, mostly in `PlatformImpl`.
Much of this logic deals with optional features or platform-specific theme modifications. Another piece of logic deals with accessibility themes: on Windows platforms, both Caspian and Modena have high-contrast stylesheets that are added or removed in response to OS theme changes. Apart from the fact that high-contrast detection only works on English locales (and there's no simple fix, see JDK-8185447), a major downside of this implementation is that it special-cases the built-in themes and doesn't offer enough extension points to make it easy for developers to create new rich themes. I think this can be very much improved by making themes a first-class citizen in JavaFX, and removing the special handling of the two built-in themes. Here's my draft PR of this feature: https://github.com/openjdk/jfx/pull/511 This PR adds a new interface in the javafx.application package: public interface Theme { ObservableList<String> getStylesheets(); void platformThemeChanged(Map<String, String> properties); } An implementation of this interface populates the list returned by `Theme.getStylesheets()` with its collection of stylesheets. Optionally, it can listen to the platform theme properties reported by `Theme.platformThemeChanged(...)` and modify its list of stylesheets accordingly. Platform properties are platform-specific key-value pairs, and theme implementations are responsible for interpreting the values. For example, on the Windows platform, a theme implementation might listen to the "Windows.SPI_HighContrastOn" property in order to detect whether or not a high-contrast stylesheet should be used. The `Theme` class documents all currently supported platform properties. A theme is activated by setting it as the user-agent stylesheet of the application, where "theme:" is a custom URI scheme: Application.setUserAgentStylesheet("theme:com.example.CustomTheme"); The existing constants `Application.STYLESHEET_CASPIAN` and `Application.STYLESHEET_MODENA` are simply redefined to their new `Theme` implementations ("theme:com.sun.javafx.application.theme.Caspian" and "theme:com.sun.javafx.application.theme.Modena"). Because theme-specific code is now located in its corresponding `Theme` class, I've been able to clean up `PlatformImpl` so that it doesn't special-case the built-in themes anymore. Going forward: If we add support for loading stylesheets from data-URIs (similar to https://github.com/openjdk/jfx/pull/508), we can then fix the high-contrast themes by reading the system colors reported back by Windows, and creating an in-memory stylesheet for these colors within the `Modena` implementation. This feature will also allow developers to create themes that dynamically adjust to OS-level accent colors or dark mode settings. For example, a `Theme` implementation can detect dark mode on Windows 10 by listening to the "Windows.UI.ViewManagement.UISettings.ColorValue_Background" and "Windows.UI.ViewManagement.UISettings.ColorValue_Foreground" properties. I'm looking forward to any opinions on this.
