Default Theme - Custom - Multisite

Hello

I asked this question before, but I don' t think I was clear, so I will try again.

I want users after signing up to be presented with only one theme and they will not be able to choose themes from the dashboard, so they will only be able to customize etc.

I wan to use the Fixer theme for this purpose, but to add my own images, text, menus etc., which will make it easier for new users to understand the functionality I am providing.

What I would like to know is how do I edit the Fixer theme and then make this the default theme across the network? I will not be using the theme myself.

Your replies appreciated.