Update readme.md

readme.md

@@ -46,6 +46,14 @@ Import the `github-markdown.css` file and add a `markdown-body` class to the con
 
 If you want code syntax highlighted, use GitHub Flavored Markdown rendered from [GitHub's `/markdown` API](https://docs.github.com/en/free-pro-team@latest/rest/reference/markdown).
 
+There are 3 themes provided in this package:
+
+- **github-markdown.css**: (default) Automatically switch between light and dark through [`@media (prefers-color-scheme)`](https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme).
+- **github-markdown-light.css**: Light only.
+- **github-markdown-dark.css**: Dark only.
+
+You may know that now GitHub supports more than 2 themes including `dark_dimmed`, `dark_high_contrast` and `colorblind` variants. If you want to try these themes, you can generate them on your own! See next section.
+
 ## How
 
 See [`generate-github-markdown-css`](https://github.com/sindresorhus/generate-github-markdown-css) for how it's generated and ability to generate your own.