“Custom Admin CSS” : String quotes are escaped

Hi!

Just tried to add some CSS to admin interface via the custom admin css option, but some characters are escaped, and so the CSS becomes invalid after saving

This :

#toplevel_page_some_plugin .wp-menu-name::after {
  content: 'Some Text';
}

Becomes

#toplevel_page_some_plugin .wp-menu-name::after {
  content: \'Some Text\';
}

Same behavior with double quotes

Thanks!