I made a Firefox addon that replaces the styles on Lemmy instances with something more modern. It works with Lemmy 0.17 and 0.18. It supports both light and dark theme.

Addon: https://addons.mozilla.org/firefox/addon/lemmy-modern-ui-theme

Repository: https://github.com/pkrasicki/lemmy-modern-ui-theme-addon

Screenshots

I also released the CSS files - they can be used as a theme on a Lemmy instance. Tell your instance admin to give it a try: https://github.com/pkrasicki/lemmy-modern-ui-theme-addon/releases