I don't see how this is different from any other memory corrupting bug in, say, a library.
From a technical standpoint it is not.
However, a theme is a library that is loaded into your application by the end-user. Even if he is not particularly aware of doing so. The application programmer has no choice in the matter and cannot really test with all kinds of themes and all kinds of versions of them. But the resulting crashes are still going to be blamed on the application and poor me. Morten