Check the RDF... It probably has a max version limit of Pale Moon 28.*Lunokhod wrote: ↑2019-12-20, 00:52Upgrading to the latest unstable Pale Moon 29.0.0a1 for Linux today (.xz !), a message was displayed that Photonic theme was incompatible and had been disabled. It was 2.2.0 Photonic which seems the latest. This suggests Photonic might need updating in due course before the stable release gets to this point.
Pale Moon development builds recently had a version bump to 29... so, you can try changing the RDF yourself and seeing if it still works.