Hiding .postHeaderShortText but keeping visible on Facebook/Mastodon

In the “List View”, I see that articles contain both the headline/subject and a snippet of the article content:

I prefer a less “cluttered” view and I don’t mind adding my own custom CSS, such as setting:

.postHeaderShortText {display: none;}

So I can get this nice clean view:

I think this looks really nice. But I have run into a problem: on Facebook or Mastodon feeds, which don’t have a “title/subject”, that CSS identifier seems to be the same that is used to show the post content.

As a result, I’m making those invisible, as in this example from Mastodon:

I had the same issue on Inoreader, but they had two different types of CSS selectors for that content. Thus I would display:none the side text by default, but if it was associated with a microblog like Mastodon or FB, I would display: block !important that selector. This let me see the Mastodon type posts while hiding the snippet from the main RSS articles.

Is there any way to achieve this on BazQux? I’m not a developer and I’m not seeing any differentiation I could use to make some simple CSS rules like this.

Appreciate any help from the community! New user here and so far enjoying it.