Examine source code of First_Theme

{
  "theme": {
    "images": {
      "theme_frame": "header.png"
    },
    "colors": {
      "frame": "#000000",
      "tab_background_text": "#ff0000"
    }
  },
  "version": "2.0",
  "name": "First_Theme",
  "manifest_version": 2,
  "description": "just for my own firefox"
}