This repository provides a boilerplate that takes care of all the heavy-lifting needed by dynamic themes with setting up WebExtension listeners, hooking with other APIs, … to let you focus on the dynamic theming part.
In the config.js file, you will find everything you need to get started: the rules, the default theme setting, the theme definitions and some extra configuration.
In the properties.js file, you will be able to add new properties, that can be used in rules.
You can run the extension using the web-ext tool: web-ext run or by temporarily loading it into about:debugging.