EDITED 08/10/2022
I have 2310 themes (as of this edit) available on Mozilla yet only 8 of my themes are available to use in Thunderbird. The Brave theme shown in this tutorial isn't even available for me to use anymore. I have no clue as to why that is. And I've not been able to find any answers either. So more than likely this tutorial is irrelevant now.
In order use a Firefox Theme in Thunderbird, you have to install it through Thunderbird. So you want to open Thunderbird then click on TOOLS. From there go to ADD-ONS.
If you are logged into your Firefox account in Thunderbird, you should see a profile area (as shown below) in the Add-ons Manager window that opens. If you are not logged in, you can login and then click on MY PROFILE. That will take you to your page with your available Themes shown.
