How does sage remove the style.css from wp_head()?
I tried digging into the theme but couldn’t find anything.
Surprisingly, you’re not the first person to ask this… apparently it’s something that’s just assumed that is enqueued by default. But no… you have to manually enqueue it: https://github.com/WordPress/WordPress/blob/master/wp-content/themes/twentyfifteen/functions.php#L233
Sage just… doesn’t enqueue it.