[Ultimate Branding] Restore Customizer Additional CSS to subsite admins

Customizr 3.5.5 removed subsite admins access to the Additional CSS option. This article explains the reasoning: https://github.com/presscustomizr/customizr/issues/860

I would like to see an option in Ultimate Branding to restore that capability to subsite admins. The article above links to a plugin that gives this capability.