I am not used to CSS syntax so it would be amazing if you add a lot of example config.yml files. Also, try adding config.yml files for the GIFs you are displaying in your README.md. They look fantastic. I have spent hours trying to get perfect GIF but no luck.
Users can set a different url in their config or as an env variable: https://asciinema.org/docs/config. We currently support this, but we don't change the hostname in messages to the user. If one is set, we should use the url's hostname rather than hardcoding asciinema.org.