Examine source code of Ruler pixel numbers

{
  "theme": {
    "images": {
      "theme_frame": "pixel_ruler.png"
    },
    "colors": {
      "frame": "#000000",
      "tab_background_text": "#000000"
    }
  },
  "version": "2.0",
  "name": "Ruler pixel numbers",
  "manifest_version": 2,
  "description": "This is the pixel ruler"
}