Examine source code of Custom

{
  "name": "Custom",
  "manifest_version": 2,
  "version": "1.0",
  "theme": {
    "images": {
      "theme_frame": "tumblr_mbudn3icf81rhztfto1_1280_efccb19555711dd6b67dfd928db74d00_raw.jpg"
    },
    "colors": {
      "frame": "rgba(108, 135, 199, 1)",
      "tab_background_text": "rgba(0, 0, 0, 1)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{b0437e46-bc13-4a1b-82f3-702aef98bf68}"
    }
  }
}