Multisite: cloning, but also updating too

My requirements are: 1 main website and 100 subsites

All subsites will have same themes, plugins, pages, menus, sidebars - everything

Only posts in each subsite will be different.

How do I manage this?

NS Cloner - but what if I need to make a change in the future to the theme or widget or something - 100% this will happen - what do I do then?

Thanks in advance