It's up to the theme developer to update their add-on to be compatible with TB, but in the meantime you can make the change in install.rdf as mentioned here. It works, for the most part, here in TB 60.2.1/W10.

Well I managed to find a backup copy of Thunderbird version 52.7 and went back to that version without difficulty. Walnut2 add-on now works fine. In the future I will wait before ever updating to a newer version of Thunderbird to make sure my add-ons will work.

montana7 said

Well I managed to find a backup copy of Thunderbird version 52.7 and went back to that version without difficulty. Walnut2 add-on now works fine. In the future I will wait before ever updating to a newer version of Thunderbird to make sure my add-ons will work.

Future versions of Thunderbird will simply not support legacy add-ons at all. So unless their developers update the themes and add-ons for Thunderbird 60 and again for the following version, they will not move forward. Much of this change is being forced on the Thunderbird community by Mozilla, but that does not escape the fact ALL add-ons require an update.