Welcome to the world of WordPress Multi Site. Here are some helpful articles and plugins to simplify the setup and management of a WordPress Multi Site Network.
Themes:
Best Multi Site WordPress Themes
Helpful Articles:
How to Install and Setup WordPress Multisite Network
WordPress Codex: Create A Network
WordPress Codex: Multisite Network Administration
The Complete Guide to Creating a WordPress Multisite Installation
Changing Themes on a Per Site Basis
Helpful Plugins:
Multisite Clone Duplicator
WordPress MU Domain Mapping
Multisite Enhancements
Network Privacy
Multi Step Form
Multisite Directory
WP Multisite User Sync
Multisite Plugin Manager
Multisite Widgets Context
WP Multi Network
Wordfence for Multisite