How can you force a stylesheet reload in Drupal7?
You can do this with drush

$ drush help cache-clear
Clear a specific cache, or all drupal caches.

Arguments:
 type                                      The particular cache to clear. Omit 
                                           this argument to choose from        
                                           available caches.                   


Aliases: cc


So you can do

drush cc css-js


and this will rescan the cached CSS and JS files, nuke them if they are stale, and generate a new key that gets appended to CSS and JS URLs.