Shows and stores your favourite colour, in storage.sync in the extension's about:addons page.

Demonstrates:

• storing data with storage.sync
• reading data from storage.managed
• creating an options page and opening it with runtime.openOptionsPage()
• best practice for supporting automatic theme switching (dark theme) in the options page

To have Firefox read data from storage.managed, create a file with this content:

{
  "name":  "favourite-colour-examples@mozilla.org",
  "description": "ignored",
  "type": "storage",
  "data": {
    "colour": "management thinks it should be blue!"
  }
}

Name the file "favourite-colour-examples@mozilla.org.json" and follow the instructions in the storage.managed documentation to make the file discoverable by the extension (on OS X and Linux this means copying it to a specific location, while on Windows it means creating a specific registry key pointing to the file's location).