Examine source code of Twilight...

{
  "theme": {
    "images": {
      "theme_frame": "TwilightH2.jpg"
    },
    "colors": {
      "frame": "#000000",
      "tab_background_text": "#ffffff"
    }
  },
  "version": "2.0",
  "name": "Twilight...",
  "manifest_version": 2,
  "description": "full moon, sunset, trees, hill, twilight, evening, birds, night, purple sky, pink sky, silhouette"
}