Hello,

There's no change for theme packs.

The theme pack is not integrated in the new Calendar module (not yet, as i've planned soon a full refactory of the theme pack system).

But it is still available in the deprecated calendar module (which is still maintain for now, even if deprecated).
And only in the depreacted module, your custom theme will work, as the new module is totally a new code, with major differences.

Hope this answers your question.

Best regards,
Cyril