Since MediaWiki can be installed on a domain where other applications or JavaScript scripts may be installed it would be good to put the LocalStorage key in a namespace by prefixing the key with mw or mediawiki or something, followed by a dot (the . character).

Such as mw.dark-mode = "true", or mw.prefers-color-scheme = "dark".

Get config:

const scheme = localStorage.getItem('mw.dark-mode') ?? 'light';

Set config:

localStorage.setItem('mw.dark-mode', 'dark');

A constant could also be used to refer to the key.

const KEY_PREFERS_COLOR_SCHEME = 'mw.prefers-color-scheme';

In T221897#7140324, @Frap wrote:

Since MediaWiki can be installed on a domain where other applications or JavaScript scripts may be installed it would be good to put the LocalStorage key in a namespace by prefixing the key with mw or mediawiki or something, followed by a dot (the . character)...

This was done in rEDKM35855e4111452c163724350c44362ab6a731c8fe (with the persistence part in rEDKM0224fa679de8bef0fddadb46e1b4b25550f6b0f5). Most of these tasks just have not been closed since Community Tech abandoned the extension.

In T221897#7141002, @R4356th wrote:

Most of these tasks just have not been closed since Community Tech abandoned the extension.

Wait! What? Is this extension abandoned? Why? I need dark mode!

@Frap: Please move general discussion to https://www.mediawiki.org/wiki/Extension_talk:DarkMode . This ticket is specifically about implementing "Allow users to switch Dark Mode on/off", not for general discussion. Thanks a lot!