Choose a maintained theme from a trusted source and confirm its requirements. Check the licence for use across the intended sites. Back up the network before adding or updating shared theme code, and test the theme on a non-production subsite with representative plugins and content.

1. As a Super Admin, open Network Admin > Themes.
2. Install the approved theme if it is not already present.
3. Use the network-enable action to make it available to all sites, or enable it only for a particular site through that site's Themes tab in Network Admin.
4. Open the chosen subsite's Appearance area and activate the theme there when that site's owner is ready.
5. Review the subsite's templates, navigation, widgets or block-theme settings after activation.

No. Availability and activation are separate. Each site retains its active theme until it is changed through an appropriate site-level workflow. This allows an agency to approve a small theme library without forcing every customer site to look identical.

All sites using that theme share its installed code. A file change or theme update can therefore affect several sites. Site Editor customisations and many appearance settings are stored for individual sites, but that does not isolate them from a shared template or stylesheet code change. Use a controlled development and deployment process.

Inventory which sites currently use the theme and any child themes depending on it. Disabling availability does not necessarily switch an already active site to another design. Do not delete its files until those dependencies have been resolved and backups exist. After an update, check a sample of different sites rather than testing only the network's main site. Keep a record of the theme version and the affected sites so a problem can be traced to the correct shared component.

• Adding a new site to an existing WordPress Multisite network
• Choosing network activation or site activation for a plugin
• Adding an existing network user to another subsite
• Planning backups for a WordPress Multisite network

• WordPress: Admin
• WordPress: Administration
• WordPress documentation