Adding an additional CSS without the editor

Hi all,

I changed the theme of my site to SmartCube: the final result is that I’ve lost some items under the ‘Appearance’ menu.

Now I only have ‘Themes’ and ‘Editor (beta)’ items: some useful links are lost, such as ‘Menu’ or ‘Additional CSS’.

I’d like to insert some custom additional CSS tag but I’m not able to.

Any idea, please?