I have a fix for the visual appearance with Yaru (aka community theme), @kenvandine please consider merging it.
@1bbrazie: the calendar add-on seems to be bundled with the thunderbird snap indeed.
@Lvaskz: the snap doesn’t ship any language pack. I tried adding a language pack but I didn’t manage to get thunderbird to pick it up (even though it was found and exposed in the extensions UI). This requires more investigation.