* `WP_Theme::network_enable_theme()` can be used to enable a theme or array of themes on a network.
* `WP_Theme::network_disable_theme()` can be used to disable a theme or array of themes on a network.
* Use these new methods in the network admin vs direct `update_site_option()` calls.
* Add tests.
Props igmoweb.
Fixes#30594.
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