Using .site-content container clearfix didn’t work because those are actually three separate CSS classes, so you’d have to use only one - for example .site-content. However, it looks like .site-content is too big, as it includes the website’s sidebar as well. You may already know this but in Firefox and Chrome you can right click anywhere on the website and use the Inspect option to look at the source, and clicking on a section of the source highlights the corresponding section of the website and this will help you find exactly the CSS class you’re looking for. I did this on a couple articles from Humble Bundle and found a couple of options:
.post: This includes only the content of the post, excluding the title and the image.
.site-main: This includes the title, author, image and the content.
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Another useful tool in FreshRSS I forgot to mention is “CSS selector of the elements to remove”. You can use it to remove certain section from the full article, I’d recommend removing .sharedaddy and .entry-footer (the sharing links at the end of the article), and also .entry-header if you use .site-main as the CSS selector for the full article (.entry-header is the title of the article, but FreshRSS already fetches it from the RSS feed so you don’t need it in the body of the article as well). You can remove multiple sections by using a comma-separated list of CSS classes to remove: