On Rails 3 I got into the habit of keeping a separate admin stylesheet (app/assets/stylesheets/admin.css), incorporating bootstrap/foundation and then in production.rb I'd have something like:

# in config/production.erb
config.assets.precompile += ( admin.css )

I could then run:

RAILS_ENV=production bundle exec rake assets:precompile

And this would generate admin.css and admin.css.gz.

But this seems to be broken in Rails 4.0.0.beta1. It might be a sprockets issue, so pardon me if that's the case.

Now, no matter what I put in my admin.css file, and no matter what I add to production.rb
the admin stylesheet is never precompiled.

Rails 4 example app
Rails 3 example app

Steps to reproduce:

1. Clone sample apps bundle install
   NB: sample apps were used to report another issue, so there is a bit of left-over detritus in there, just ignore it
2. Starting with the Rails 4 example app, run:
   RAILS_ENV=production bundle exec rake assets:precompile
3. Inspect public/assets
4. Observe that the admin stylesheet has not been precompiled
5. Repeat steps 2 and 3 using Rails 3 sample app
6. Observe that admin stylesheet has been precompiled

For now I'm getting round this by lobbing everything into my application.css

This happens with js assets too.