I'm experiencing the same issue after upgrading from Bookworm to Trixie! The Custom debian-breeze theme user config is not upgraded properly for Trixie's variant, and causes a white background! Even reinstalling sddm doesn't solve the problem. The only fix is that the user goes into the theme's config and manually tweaks the background path, which imho is critical bug as for now that Trixi is about to be released...
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