Have you tried simply using Stylus with a custom style? I've been pretty happy with the setup. No performance issues.
Stylus is great when Darkreader breaks on certain sites. But when it works there are no substitutes e.g. navigating HN using the light setting in Darkreader offers a degree of customisation to create a scheme and reduce eye fatigue, without too much effort. Whereas, it can be hit and miss when trying various themes/stylesheets etc.
a lightweight alternative to dark reader is this one: