Themes for 56.0.2? - Installation aborted because the add-on appears to be corrupt.
Hotfix for Firefox bug 1548973 (armagaddon 2.0) mitigation is installed.
All themes tried fail with "Installation aborted because the add-on appears to be corrupt."
Additional System Details
- Shockwave Flash 32.0 r0
- User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:56.0) Gecko/20100101 Firefox/56.0
Themes are currently installed as an extension (XPI file) and this isn't compatible with older Firefox versions. I can load a theme that I tested in Firefox 60 ESR, but get the same error you reported in Firefox 56. I also tried the about:debugging page to load a temporarily extension that reported that it wasn't a valid extension, so you are out of luck with a direct install.