add @import 'ladda'; to make it work in Rails

I have rails 4.1.4

Without @import 'ladda'; in ladda-theme.scss I see this error

(Undefined mixin 'buttonColor'.
  (in /Users/user/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/bundler/gems/ladda-rails-5c51ac8bc99f/vendor/assets/stylesheets/ladda-theme.scss:41)):