I wrote a chrome extension that does just this! https://chromewebstore.google.com/detail/line-highlighter/nffehhefkilbinmemhnhepadbeadnfep

You can see the technical implementation and source code here if you're still interested in doing it yourself: https://github.com/kylechadha/line-highlighter?tab=readme-ov-file#technical-implementation. It's open source, so enjoy!