I had no problems installing and using the Nuvola and Phoenity Shredder complete themes using Thunderbird 60.0 with Windows 10 Home. However: * The Monterail theme still looks like the default theme. If I exit version 60, install 52.9.1 in a different directory and run it, the Monterail theme looks fine. If I then exit and switch to 60, the theme looks fine. * The Metal Lion Sea Monkey uses the default theme on the main message window. The rest of the windows use the correct theme. If I switch to 52.9.1 the theme looks right in every window. If I exit and switch to 60 it looks right in every window. * Same for Metal Lion Silver Sea Monkey * The add-on manager in version 60 claimed Classic TB2 was corrupt. I was able to install it in 52.9.1. If I exit and switch to 60, the theme looks fine. In each case I downloaded the complete themes using a browser and then tried to install them via the Thunderbird 60 add-on manager using "install add-on from file". The two versions of Thunderbird are in different directories and use the same profile. There appear to be only six complete themes that support version 60(or later). Four of them required me to somehow use a earlier version of Thunderbird in order to get them to work with version 60. This makes me suspect there is either a bug in Thunderbird 60 or something important is not communicated clearly in the add-on guide for 57-60.
No TB 52 is needed, install it a second time on TB 60 without deinstalling it. I can't say where's the problem but I think it is because m-c removed code they don't use anymore that is needed for full themes to install correctly.
I found a workaround for Monterail theme: 1) Go to the "extensions" folder located in the application data folder (%appdata%\Thunderbird\Profiles\[...].default\extensions in Windows), here there should be the .xpi file with the theme 2) While Thunderbird is opened in background, delete the file 3) Restart Thunderbird and reinstall the theme 4) Restart Thunderbird again, the theme now should work
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