Create MU child theme without FTP

Hello,

I'm setting up a multiuser site using twenty fifteen, and I want to create a child theme. I currently do not have FTP access to the server (trying to get, but not sure when).

Is there a way to do this without using FTP? I tried using the wp-filemanager plugin, but it appears to only work for sites (not the network admin level), and I can't go below a specific directory.

Any advice on creating a network child theme without using FTP is greatly appreciated.

- Peter