Examine source code of simple background + theme

{
  "name": "Simple Theme",
  "manifest_version": 2,
  "version": "1.0",
  "theme": {
    "images": {
      "theme_frame": "Untitled design(1).png"
    },
    "colors": {
      "frame": "rgba(229, 230, 232, 1)",
      "tab_background_text": "rgba(141, 166, 169, 1)",
      "toolbar": "rgba(51, 48, 48, 1)",
      "bookmark_text": "rgba(141, 166, 169, 1)",
      "toolbar_field": "rgba(249, 244, 225, 1)",
      "toolbar_field_text": "rgba(141, 166, 169, 1)"
    }
  }
}