Don't bother trying to install the Ultraviolet gem, just setup RestClient and go to town:
Find the themes in /stylesheets/themes, like /stylesheets/themes/mac_classic.css.
POST data to /api
curl "http://ultraviolence.heroku.com/api?s=css&l=1" -d '.abc {}'
RestClient.post "http://ultraviolence.heroku.com/api?s=css&l=1", %(.abc {})
?s or ?syntax - specify syntax?t or ?theme - specify textmate theme (defaults to mac classic)?l or ?line_numbers - set to '1' to show line numbers