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.
Oh awesome, thank you! My docker compose template must've been from before they added the extra_templates volume for lemmy-ui which is why I couldn't find it at first.
I've set it as the default theme for my instance, and provided a link on the site's sidebar if that's alright with you!