This was fixed recently in the latest version of the Marfa plug-in. It looks like you do have the latest version installed, but you also have a custom theme that has its own style.css template. That is most likely using the old CSS.
So to fix it on your site, I think you’ll want to either remove your custom style.css template, or edit it to add the changes that were added to the plug-in. The change in this diff (Fixed conversation margins/padding from other recent CSS changes. · microdotblog/theme-marfa@9d83176 · GitHub) was basically to add “padding: 0” because another recent change added padding around blog post images, which also affected books and conversations unintentionally.