Install the following plugin to use custom CSS:
https://raw.githubusercontent.com/eth-p/unraid/master/release/eth-p.unraid.custom-css.plg
Run:
curl -J -L "https://gist.github.com/eth-p/557cc699c4966442edb8ee56dcc74126/raw/custom.css" -o "/boot/config/plugins/eth-p.unraid.custom-css/custom.css"