Examine source code of Mac OS X Matched Grey

{
  "theme": {
    "images": {
      "theme_frame": "header.png"
    },
    "colors": {
      "frame": "#f1f0f1",
      "tab_background_text": "#000000"
    }
  },
  "version": "2.0",
  "name": "Mac OS X Matched Grey",
  "manifest_version": 2,
  "description": "Grey gradient to match Firefox with Mac OS X (Yosemite and higher). It matches the gradient of the Mac desktop windows toolbars as closely as possible. UPDATE to FF 58: the blue thin line accentuating the tabs now exactly matches the theme’s color."
}