Are all those users on Apple devices?
By default, a custom theme applies only on the current device, unless you check this box manually:
When that happens the theme preference is saved locally in the device using a cookie.
On Apple devices native browsers, those cookies are removed after 7 days:
Workarounds are:
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Apply theme to all devices, so we save that on the server
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Never fail to visit the site for 7 days
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on MacOS you can use another browser. Not an option on iOS
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on iOS you can install the Discourse PWA with “Add to Homescreen” which bypasses the normal 7 days counter for a different one.