It has happened to my two times already. Using the excellent front-end framework Semantic-UI for development is great, but when I have to deploy, is like I never used it!

You can find more astronauts in space than resources on how to fix this problem, but I managed to find this rather obscure article on precisely that.

It basically boils it down to:

Find config.assets.compile and true it up

#config/environments/production.rb
config.assets.compile = true

compile them locally

RAILS_ENV=production bundle exec rake assets:precompile

all the git usual suspects

git add .
git commit
git push heroku master

I've just deployed 1 minute ago and it's working.

So, thanks Ellen!