Before switching to Tree Style Tab extension I was quite happy with customizations from https://github.com/Izheil/Quantum-Nox-Firefox-Dark-Full-Theme https://gi

Before switching to Tree Style Tab extension I was quite happy with customizations from https://github.com/Izheil/Quantum-Nox-Firefox-Dark-Full-Theme https://github.com/Izheil/Quantum-Nox-Firefox-Dark-Full-Them... - they use Firefox beta to adjust css so once new Firefox version is released everything is ready from the get-go.