It's pretty close to the default theme with Yaru (https://github.com/ubuntu/yaru) prior to the version 19.10.2.
https://addons.mozilla.org/firefox/addon/yaru-hybrid/
The active tab and the toolbar have a light color while inactive tabs
have a dark color.
For comparison, here are the default themes with Yaru 19.10.2.
- Yaru (light)
- Yaru-dark
Firefox Color (https://color.firefox.com/) is great for creating your own theme, but it doesn't expose all available options for good reason. Just wanted to specify the color of the border between the toolbar and a web page not to be too distinct like the following screenshot.
