Adding theme in WordPress multisite install is different than adding it from a single site. So, here we will describe adding theme in WordPress multisite install.
1. Open network admin dashboard of multisite.
2. Open themes page in the network admin dashboard of multisite.
3. Upload and install a new theme here. Click Network Enable to enable theme in all websites of network.
4. Open the themes page for the subsite of the network.
5. Activate the theme you want.
The installed theme can be used in any of your sites added in your multisite install, which is only one and used in many different sites.