Examine source code of www.youtube.com

{
  "name": "My Theme",
  "manifest_version": 2,
  "version": "1.0",
  "theme": {
    "images": {
      "theme_frame": "12.jpg"
    },
    "colors": {
      "frame": "rgba(42, 104, 130, 1)",
      "tab_background_text": "rgba(26, 2, 2, 1)",
      "toolbar": "rgba(158, 44, 44, 1)",
      "toolbar_field": "rgba(54, 19, 19, 0.16)",
      "toolbar_field_text": "rgba(10, 2, 2, 1)"
    }
  }
}