When I click on “Add to Thunderbird” (via Firefox browser) it doesn’t do anything. When I search for it directly in Thunderbird get add-ons it says not found.
I installed it on another PC yesterday just fine, bizarre.
From there just search for the title or key words. I just did a black glass search and found the one you were talking about. From there you can click the add button in the bottom left hand corner of the design area when you hover over it. Or you can click on the thumbnail and go to the design page.
I hope this helps. I had tried to find the design by doing an initial search without going to the Featured Designs link and it didn’t find anything that way. So that may have been what you were experiencing.