Well I couldn’t be arsed hacking up the code to rename the directory, so I used mod-rewrite to change the files I wanted.
This will get you started, place the code in your root htaccess files.
# Rewrites the stylesheet to root
RewriteRule ^style.css$ /wp-content/themes/[INSERT.YOUR.THEME.NAME.HERE]/style.css [L]
Now visit https://yoursite.com/style.css —it’ll be the same as /wp-content/themes/[INSERT.YOUR.THEME.NAME.HERE]/style.css xD
Use as you will.