This is a theme component that will add a button to the post menu. When you click the button, you’ll get a modal / popover with the the raw (markdown) version of the post.
There’s only one setting and that setting is
This setting allows you to set a minimum trust level the current user needs to have before they can see that button. It defaults to TL0. Anons will never see it.
Follow the theme installation guide and add the theme component to your active themes.
If you’re new to Discourse themes, you can learn more about them here.