In order to get assets to work on Heroku (js, css and images) we need to precompile them.
The Rails asset pipeline provides an assets:precompile rake task to allow assets to be compiled and cached up front rather than compiled every time the app boots.
If you have made any changes to the css or js files or if you have added any images to the project, please run RAILS_ENV=production bundle exec rake assets:precompile to precompile assets before you make a PR.
Remember to add them to git with git add public/assets -A or git add . -A and make the commit and submit the PR.