Its the BEST appearance extension ever. Most themes for Firefox are tolerable, not beautiful. This ext provide one of the best colors and borders scheme like mine - Borderless. Its feels really good with different color for each site, the most beautiful Ive seen from themes.
Thank for improving user experience as much as possible (for me ig)

Good extension. Would be cool if Mozilla built this in Firefox

I have no idea how this isn't built into Firefox yet.