This is a sync-enabled Firefox extension. Custom Style Manager allows domain-level CSS styling for any website you visit. Your changes will sync across all of your logged-in Firefox browsers.
The host rules JSON object structure can be found at least in popup.js > setRules() > tempStorageObject.
It should be better documented, but if it's not then you can find it there.
Parsing the JSON rule object as sync.storage objects currently are, and adding the correctly-formatted version to sync.storage should be sufficient to make this work.