CSS Code Overrides Not Working

I have really enjoyed discovering using your plugin since as an old-school developer I like being able to comment code in a separate own block.

:root {
  --markdown-comment-color: #B4473C;
  --markdown-comment-font-size: 15px;
  --markdown-comment-line-height: 1.8;
 }

However, I dropped your suggested code in my custom CSS editor, using the Twenty Sixteen theme, and I get the error Expected Rbrace when leaving the — before the properties, and Unknown Property when I remove them.

I’m absolutely ready to give you a 5* review once I find out how to customize the comment styling.

Thoughts?

The page I need help with: [log in to see the link]