I was skeptical when a recent update required all-site permissions, and it turns out the extension has now been suspended for violating Mozilla's policy. While I'm not sure why the developer made that choice, it is truly unfortunate.
Could they have been unauthorizedly misusing user data? Personally, I don't believe full website access is necessary just to change themes. It actually functioned perfectly well without those privileges in the past.
Either way, it's a pity that we've lost yet another wonderful extension.