Ah, I knew that they'd dropped support for Sol Calendar a long time ago, didn't know whether this one would be the same. Yeah, not even in the Play Store any more, so not easy to test the "dark mode" theory (there's a 2017 version in apkmirror, but I don't really want to set up an email app in order to test a bug when there's no possibility of a fix coming). But yeah, an incompatibility with an unsupported app sounds possible. I'd wondered whether a "night light" feature might also do this, since toggling display mode doesn't help, but such things are typically system-wide and don't require any action on the app dev's part so that's unlikely to be it.