Examine source code of Gnome3.20-background

{
  "theme": {
    "images": {
      "theme_frame": "header.png"
    },
    "colors": {
      "frame": "#ffffff",
      "tab_background_text": "#000000"
    }
  },
  "version": "2.0",
  "name": "Gnome3.20-background",
  "manifest_version": 2,
  "description": "For a nice integration with the default Gnome 3.20 background desktop."
}