They are already approved and available at AMO, but using the function “Find Updates” in Addons Manager, I get the message “There was an error downloading Silvermel.” Can someone help me with this issue?
My guess is the problem relates to the multi-package XPI using the same ID as one of the internal packages. I’m not sure how that can be fixed. I suppose you can create theme listing for the theme package and that should at least update that part.
I don’t think is it worth to fix the multi-package thing on my side, since the support for multi-packages will be dropped in FF 53. I can’t understand this decision, since in FF 57 XUL based extensions (and themes of course) will not work anymore. This decision was taken also without communicating developers about it. There is also no advice about this decision at the official documentation: https://developer.mozilla.org/en-US/docs/Multiple_Item_Packaging
Yes, dropping support for multipackage XPIs wasn’t announced well. Unfortunately there’s a lot going on in the way of compatibility, and there are only a handful of people still using them. I included a note about it in the 53 compatibility post.