Examine source code of Firefox 57 Dark

{
  "theme": {
    "images": {
      "theme_frame": "header.png"
    },
    "colors": {
      "frame": "#282828",
      "tab_background_text": "#ffffff"
    }
  },
  "version": "2.0",
  "name": "Firefox 57 Dark",
  "manifest_version": 2,
  "description": "Just another dark theme for Firefox. What else do you expect?\n\nFirefox icon recolor done by me. If you want to use it for your own [whatever], please give me credit."
}