Examine source code of Portal 2

{
  "name": "ezgif.com-resize",
  "manifest_version": 2,
  "version": "1.0",
  "theme": {
    "images": {
      "theme_frame": "ezgif.com-resize(1).png"
    },
    "colors": {
      "frame": "rgba(75, 53, 97, 1)",
      "tab_background_text": "rgba(138, 211, 219, 1)",
      "bookmark_text": "rgba(97, 190, 230, 1)",
      "toolbar_field": "rgba(230, 129, 235, 0.29)",
      "toolbar_field_text": "rgba(204, 118, 195, 1)",
      "tab_line": "rgba(233, 120, 235, 1)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{44e5cb9c-0480-424a-bab2-58fa699f580c}"
    }
  }
}