invalid command code u

[badger codes, the endling]

Hi EliverLara. Thank you for your lovely colors. I'm not quite there yet. Running into the issue below. Any thoughts as to a possible fix? Thank you!

kellyfletcher~$ sed -i '1s/^/@import "firefox-sweet-theme/userChrome.css";\n/' userChrome.css
sed: 1: "userChrome.css": invalid command code u