For some reason, I can't access any files in the public dir. Not found error. I'm not putting the public part in the URL, obviously.
View the Lovers source code repository on GitHub.
Sweet. I fixed this. After reading about Sinatra Configurations, I added this line to my application class:
set :root, Lovers.root
That worked!
来源：https://stackoverflow.com/questions/4989711/how-to-deploy-a-modular-sinatra-app-to-heroku