If you search the web for “bourbon sinatra” you get a whole lot about Frank Sinatra and his drinking habits. But that’s not what this post is about.
The following is standard, from their documentation.
$ gem install bourbon # or better, add to your Gemfile and use bundler $ cd views/scss $ bourbon install
Next, import bourbon in your main scss file (as first statement).
The following is the important and the undocumented part, specifically for sinatra. You need to require the ruby libraries in your scss route.
get '/assets/css/:name.scss' do |name| require './views/scss/bourbon/lib/bourbon.rb' content_type :css scss name.to_sym, :layout => false end