Perhaps a silly question, but can you take a look at your own user preferences, on the interface tab? There have been some changes recently with how themes and color palettes are managed in Discourse. You should see the same theme and use theme defaults for light and dark mode.
You may also like to check that other themes are deleted or not user selectable in the admin interface, just to make sure that everyone is using the same theme.
If none of this troubleshooting advice we are giving you works then you will need to dive into the code and see what’s changed in core discourse that affects your theme. If this is too hard for you do on your own then you can post in Marketplace to get paid help from an expert. Unfortunately this goes beyond the help we can give you here in Support.