Only the super administrator of a network can install themes and make them available for sites to use. You can do this in one of two ways: you can allow them for individual sites or you can network activate them. A theme can only be activated on a site if it’s been network enabled or enabled for that site.
Here’s how you install a theme and network enable it:
In the Network admin screens, go to Themes > Add New and install the theme in the same way as you would for a standard WordPress site.
On the ‘Installing Theme’ screen that appears, click the Network Enable link.
Alternatively, to network enable a theme you’ve already installed, click on Themes to go to the Themes screen, then click the Network Enable link below a theme.