Why did Mozilla dump all the Thunderbird themes? I’m very disappointed. 
That is correct. The downloads are all dumped on that page. In fact there isn’t any other place you can go for addons. That is the designated page.
You are trying to access it by its ID? I’m not sure those were carried over. Since themes seem to be available to me:
https://addons.thunderbird.net/en-US/thunderbird/addon/greenleafs-v2/?src=external-discourse
I was able to download the TT DeepDark but the greenleafs link is also showing the addon was removed. So how do I add this addon after downloading it?
I got the manual install sorted after taking the time to look. I also was able to get rid of the preview pane. Glad to be back on Thunderbird.
I searched addons within Thunderbird. If the addon repository was moved they need to update the links.