Multisite

Network-activate it and every site gets its own settings, including sites created afterwards. A reader's choice is each site's own too: in the browser, on a network of sites in subfolders, which all share one address (readers of those sites, except the main one, choose once more after updating to this version); and on a signed-in reader's account, on any network. A choice kept on an account before each site kept its own still applies on every site until the reader chooses on that site.

Network-deactivating it takes a few seconds, however many sites the network has. LiteSpeed Cache and WP Super Cache are asked once, for the whole network, and NimbleCache, switched on for the network, empties every site by itself. The other caches are asked one site at a time for about three seconds. On a large network that does not reach every site, and the sites it misses keep their cached pages, dark mode and its button included, until they expire: empty those caches yourself if that matters. A site that has also switched the plugin on for itself keeps running it. Network-activate it again and each site empties its cached pages on its first request that is not answered from the cache.