Examine source code of Bulgarian grey 2

{
  "theme": {
    "images": {
      "theme_frame": "header.png"
    },
    "colors": {
      "frame": "#000000",
      "tab_background_text": "#ffffff"
    }
  },
  "version": "2.0",
  "name": "Bulgarian grey 2",
  "manifest_version": 2,
  "description": "A solid gray theme that offers easier readability thanks to the dark grey background and highly contrasting white fonts. Optimized for tabs on bottom and visible Menu bar.\n\nFor tabs on bottom in Firefox 57+, follow the instructions here: <a href=\"https://outgoing.prod.mozaws.net/v1/b80445858f19dd9f574272ff24f473db3d34cfe27bbc0ca088755a1400bb246c/https%3A//tinyurl.com/ybnfbuv8\" rel=\"nofollow\">https://tinyurl.com/ybnfbuv8</a>\n\nFor tabs on bottom in Firefox 65, replace the chrome.css with this one: <a href=\"https://outgoing.prod.mozaws.net/v1/dc85330cd696ee7a1bf04b5341502c650e76a8ceac8b030ae84aeb833ebb7919/https%3A//tinyurl.com/yd9hb474\" rel=\"nofollow\">https://tinyurl.com/yd9hb474</a>\n\nFor custom tab bar color, add this text to the chrome.css: #tabbrowser-tabs {\n    --tab-line-color: #000000 !important;\n}"
}