What happened to all the themes on the Thunderbird site? Now there only seems to be a relatively small number.
They haven’t been converted to Web Extension themes is my conclusion.
Theme support: Only WebExtension themes are supported now. Both addons.mozilla.org and addons.thunderbird.net now provide WebExtension themes.
Can you explain what “WebExtension” themes means or are?
What does “WebExtension” mean?
I have been trying to submit some of my themes that will work with the latest Thunderbird and it is almost impossible. Actually so far for me it IS impossible. I can get it to pass all the checkmarks.
Now I can’t even sign in to the account. I keep clicking on send email and it never sends it! Frustration is the name of the day!
As of the latest update, I only see two compatible themes and two built-in themes in the app itself. This saddens me greatly, because I do not expect other existing complete themes to be updated (nobody owes the users anything), and the available 4 are unusable for me. 
Is there a way that I could possibly convert an existing theme myself for own usage? I don’t think there’s supposed to be lots of issues, it’s just a theme. Is there maybe a template for a new format of a complete theme?
You can add any of these themes (personas, not full themes) by dragging the Download Now button and dropping it onto TB Tools/Add-ons/Themes.
https://addons.thunderbird.net/EN-US/thunderbird/static-themes/nature/?sort=rating
You could imitate an existing full theme or create your own by making a userChrome.css file. Here is an example based on the Office 2003 theme:
https://support.mozilla.org/en-US/questions/1271886#answer-1282660
Out of the over 2000 themes that I have online, most of which were online before the switch to Static Themes, only 8 of them are compatible with Thunderbird now. And that makes NO sense to me.
Because a theme that I wanted to use was no longer available for Thunderbird, I decided to try to upload it directly through the Thunderbird site. However, the only option for upload was .zip, .xpi, .crx, .jar or .xml files. Because of that I recreated the theme I wanted to use using Firefox Color as it lets me save it in zip or xpi format. When I uploaded it, however, I got an error message that said, “Your add-on failed validation with 1 error.” The thing is, when I went to the link provided to see the validation report, it stated that my add-on passed validation on all points.
So what I’d like to know is can new themes be uploaded to Thunderbird at all?
Thank you for taking time to share the knowledge. I really appreciate it. 
Those “themes” are not themes, though. Those are just pictures in the header that don’t make TB any more friendly on the eyes. As I understand it, the Complete Themes are gone for good, which is a real shame.
However, if something can be saved with user css, I’ll give it a try. I have no issue with writing own css. I’ll check that thread out, thank you again!