Instructions unclear, I wrote a Firefox addon instead :D https://addons.mozilla.org/firefox/addon/lemmy-modern-ui-theme
But you should be able to take the script and easily turn it into a userscript: https://github.com/pkrasicki/lemmy-modern-ui-theme-addon
It works with both light and dark theme, but sometimes the detection fails for me and applies wrong styles. I hope those hacks will last for some time though so that we could have a modern UI. I would change more stuff, but some things are not possible.
Thanks :). Do you know where to find those older themes? It would be interesting to see how they were implemented.