Examine source code of Postbox

{
  "theme": {
    "images": {
      "theme_frame": "postbox-.png"
    },
    "colors": {
      "frame": "rgba(229,230,232,1)",
      "tab_background_text": "#000"
    }
  },
  "version": "2.0",
  "name": "Postbox",
  "manifest_version": 2,
  "description": "Postbox default theme background"
}