The default browser themes in Firefox are nice enough that I used them for years. But when one wants a change, 🤷🏽♀️
This felt different enough: Abyss, a theme inspired by VSCode's Abyss. Mostly dark blues and purples, hopefully calming.
Making your own theme is as easy as changing some values, then preparing the theme to be used by Firefox. Go ahead, check out the code and instructions for the theme on Gitlab.