Here is the source code for both the chrome and firefox extension! It's simple but it's mine. Feel free to download, fork, use, and mess with it all you want. No attribution or anything required.
Check out the community driven theme library here
IMPORTANT Default theme is made to work with Lichess' dark background.
UPDATE: You can now use your own custom colors with Prettier Lichess! Simply click on the extension icon and choose different colors that will change the entire site.
Lichess is an amazing platform that has more than just the ability to play chess, and all for free. Unfortunately I felt like it didn't show this well with it's current design, nor did it show it was a site that resides in 2021. Now with Prettier Lichess, Lichess becomes more readable, more discoverable, and prettier!