Examine source code of Ultra Dark

{
  "name": "Sombre",
  "manifest_version": 2,
  "version": "4.0",
  "theme": {
    "images": {
      "theme_frame": "BandeNoire.PNG"
    },
    "colors": {
      "frame": "rgba(0, 0, 0, 1)",
      "tab_background_text": "rgba(201, 201, 201, 1)",
      "toolbar": "rgba(0, 0, 0, 1)",
      "bookmark_text": "rgba(255, 255, 255, 1)",
      "toolbar_field": "rgba(20, 20, 20, 1)",
      "toolbar_field_text": "rgba(255, 255, 255, 1)",
      "tab_line": "rgba(255, 255, 255, 0.2)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{03f7f700-a42f-4841-8dfc-c33894bd52d3}"
    }
  }
}