I am not sure I get the benefit of looking just at one rss feed at a time.
But that said, for the longest time, firefox did just style the feed directly and I am still sad they removed this.
Maybe there is a browser plugin which just adds the style back?
A browser plugin is a pretty heavy weapon. It gives a lot of power over your browsing to whoever controls it. So I would not use one just to read rss feeds.
Greasemonkey is really great to customize websites for your [weird] usage pattern. (Tampermonkey[0] works in Chrome, Microsoft Edge, Safari, Opera Next, and Firefox)