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 none of the AWT colors are present so the limits can be tested.