Paxmod is my take on building a dark, minimalist and compact Firefox theme:
https://github.com/numirias/paxmod
It's not just a pure theme, but also adds some features such as multiple tab rows and colorful tab labels that are derived from the site's favicon.
It's currently mainly tested on Linux (although it most likely works elsewhere too), so I'd appreciate any help testing and tweaking it for other environments. Let me know what you think and if you want to contribute.
r/FirefoxCSS would love this.