At @Ali_RS’s request I’ve made a flat medieval theme for Lemur. It supports all the stock elements and some of the elements from lemur-proto. All the assets from the UI pack this is based on are included, so one can make support for additional elements by themself if needed.
For more, check it out here: GitHub - grizeldi/MedievalTheme: A medieval theme for the jmonkeynegine3 Lemur GUI toolkit