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this topic has information about how to format embedded posts:
Styling your Embedded content
If you are running Discourse 1.4.0 or greater, you have the ability to add a stylesheet for your embedded comments. Use the Embedded CSS section of the Theme editor at Admin > Customize > Themes > Edit CSS/HTML and you can add a custom stylesheet that will be served up with your embedded comments. By default we think the layout looks nice on a white background, but if your site has a unique layout you’ll want to style it yourself.