Steps to replicate the issue (include links if applicable):
Visit wikipedia anonymously - without logging in. Look at the appearance panel on the right.
What happens?:
Default appearance color setting is light instead of automatic. This forces users who use dark mode on their system to select either automatic or dark. That saves the mwclientpreferences cookie which expires after 30 days. So every 30 days anon dark mode users will be flashbanged with the light mode page despite having changed the setting already.
What should have happened instead?:
If automatic was the default appearance color setting anon system wide dark mode users would not even need to set the cookie at all and the 30 day expiry would not matter any more. Although the text and width settings would still revert back.
Other information (browser name/version, screenshots, etc.):
There was discussion about whether to use persistent local storage or an expiring cookie for these settings and in the end the cookie was implemented. Part of the reason for the cookie expiring is to prevent tracking of special combinations of settings deanonymizing anon users. Setting the default color option to automatic will fix the dark mode issue for anon users who use system wide dark mode without the deanonymization issue.
https://foundation.wikimedia.org/wiki/Policy:Cookie_statement
"mwclientpreferences 30 days Preferences Stores user preferences for client-side settings, such as font size, dark mode, and width (wide or standard). This cookie ensures a customized browsing experience for anonymous users. For more details, see the decision record."
https://www.mediawiki.org/wiki/Special:MyLanguage/Technical_decision_making/Decision_records/T333867