A browser's built-in Reader Mode (Safari, Mac and mobile, Firefox, Edge's Immserive Reader, Chrome has one behind a feature flag) can be very helpful when they work. A better solution is probably one of the browser extensions, like Stylus [0], that enable not just a single user stylesheet but the ability to have custom stylesheet's on a per-domain basis. On top that personal customization, they can load stylesheets others have already created and shared on userstyles.org [1].
[0] https://en.wikipedia.org/wiki/Stylus_(browser_extension) [1] https://userstyles.org