There are extensions that let you implement custom CSS("userstyle"), why not use those instead?
https://userstyles.org/styles/160459/hacker-news-readable-da...
If an extension goes rogue, everything you do in your browser is compromised https://www.grahamcluley.com/rogue-browser-extension/
"Stylish" browser extension steals all your internet history https://robertheaton.com/2018/07/02/stylish-browser-extensio...
My browser, the spy: How extensions slurped up browsing histories from 4M users https://arstechnica.com/information-technology/2019/07/datas...
DataSpii: The catastrophic data leak via browser extensions https://securitywithsam.com/2019/07/dataspii-leak-via-browse...
Chrome Extension Stealing Cryptocurrency Keys and Passwords https://www.schneier.com/blog/archives/2020/01/chrome_extens...
Google removes 500+ malicious Chrome extensions from the Web Store https://www.zdnet.com/article/google-removes-500-malicious-c...
The Case for Limiting Your Browser Extensions https://krebsonsecurity.com/2020/03/the-case-for-limiting-yo...