Examine source code of Pretty....

{
  "name": "Pretty....",
  "manifest_version": 2,
  "version": "4.0",
  "theme": {
    "images": {
      "theme_frame": "1B.jpg"
    },
    "colors": {
      "frame": "rgba(108, 203, 217, 1)",
      "tab_background_text": "rgba(0, 0, 0, 1)",
      "toolbar": "rgba(106, 202, 217, 0)",
      "bookmark_text": "rgba(0, 0, 0, 1)",
      "toolbar_field": "rgba(106, 202, 217, 0.07)",
      "toolbar_field_text": "rgba(0, 0, 0, 1)",
      "tab_line": "rgba(0, 0, 0, 1)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{ee7e0a20-a278-4267-9a1e-26c10b4eaa99}"
    }
  }
}