I do not think there are any "complete themes" that are compatible with Thunderbird since about V68. Nope support document says support ended with V68. https://support.mozilla.org/en-US/kb/install-themes-change-look-thunderbird#w_what-are-themes There are a couple of themes listed as addons that go close See [https://addons.thunderbird.net/En-us/thunderbird/search/?q=theme&cat=1%2C0&appver=128.7.0&platform=windows these], but the complete theme category was retired with the adoption of web extensions back around V68. What has me confused is how or where you are finding a web page showing complete themes either via browser or from Thunderbird's add-on manager I can not replicate that at all. I recall seeing it in the past, but I can not longer get such a category to show. ahh, found it, it is still indexed at google, just not visible on the main web page so a google search for "complete themes" will locate the page on which nothing is compatible with a version after 68. I guess it was left on the server for the Luddites that have not updated their Thunderbird for years.