More info: Firefox.com reUX.
More info: GitHub Enterprise reUX.
More info: Prime Video reUX.
More info: Improving LinkedIn's UX / UI.
More info: Apple.com React Redesign.
More info: Mattermost Chat UI Update.
Immediate mode UI, 9-patch and kerned Angel Code bitmap font support (vector fonts also). Button rethemes itself on mouse down (supports key binding/key-combos).
A simple test game or interactive mockup I made on overriding the AWT's paint method to retheme components. Here it is much easier to see the limitations because I didn't work with any of the AWT colors and purposely tried to push the limits.