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Sorry to hear you’re facing problems 
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Nextcloud version (eg, 20.0.5): 21.0
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The issue you are facing:
I want to disable nextcloud from adapting the system settings. It is quite annoying to have the default dark mode enabled - which really doesn’t work that well - in shared links. Without disabling the accessibility app I haven’t found a way yet, trying to edit the config.php with ‘theme’ just caused errors, but i didn’t find any source which themes are available (name of the standard theme).
Where is the information about adapting the system settings stored and can I change that value without having to do this on every update?