An alternative to the Stylus extension that the parent comment mentions is the Stylish extension, which has a "marketplace" almost exactly what you're looking for. You don't even need the Stylish extension as each theme is also offered as a userscript.
Yeah, there's that; I wasn't sure about affiliation status between userstyles.org and Stylish.
FWIW, I still recommend installing Stylus instead of Stylish - the difference is that at some point, Stylish changed owner and acquired analytics; Stylus is a fork that's free of privacy-invading antifeatures.
https://userstyles.org