Live reloads of stylesheets have been disabled for production instances, because they were causing problems when updating themes and/or Discourse core.
Fundamentally: stylesheet changes are often tied to changes in JS/HTML, so live-loading for only stylesheets is risky. Users end up running ‘new’ stylesheets against ‘old’ HTML, which can lead to some very strange experiences.
Live-loading is still enabled for development-mode Discourse instances. Ideally, you’d use that for development rather than a production site. Apart from the stylesheet reload, development-mode Ember gives you waaaay more friendly error messages than production-mode Ember.