Custom themes not showing up?

I'm trying to install this theme on my instance, installed using docker. I've added

volumes: - ./volumes/lemmy-ui/extra_themes:/app/extra_themes

to the docker-compose.yml so I could see the extra_themes folder. I've tried every combination of making folders, standalone .css, .scss in extra_themes, but I can't seem to get the theme to show up. Any help would be greatly appreciated!