Examine source code of Roblox Theme :)

{
  "name": "Roblox Theme :)",
  "manifest_version": 2,
  "version": "1.0",
  "theme": {
    "images": {
      "theme_frame": "Admin Icon.jpg"
    },
    "colors": {
      "frame": "rgba(18, 83, 235, 1)",
      "tab_background_text": "rgba(255, 0, 0, 1)",
      "toolbar": "rgba(111, 204, 45, 1)",
      "toolbar_field": "rgba(255, 230, 5, 1)",
      "toolbar_field_text": "rgba(0, 0, 0, 1)",
      "tab_line": "rgba(255, 9, 0, 1)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{4236bd7e-edf0-4ea5-8afb-4443d1fa0122}"
    }
  }
}