I am currently using it with Thunderbird 52.1.1 even if it says that it is incompatible. The problem only comes from a version number in the files A procedure is described on the maintenance forum to make it work (http://forums.mozillazine.org/viewtopic.php?f=48&t=1520265&start=1755).
I'll try to give more details:
0. The objective here is to modify the version number in 3 files to allow Thunderbird to install the theme. It's a bit annoying to have to do it, but it's totally worth it!
1. Download the file. It will have a name close to charamel-2.0.1-fx+tb.xpi
2. Open this file with an archive manager, like 7-zip or even the Explorator from Windows. Double-click or right-click and "Open with..." and select the archive manager.
3. You will have 3 files, install.rbf, charamel.xpi and silvermelxt.xpi.
4. Open install.rbf. Search for the number of the last version of Thunderbird that the current page says it is compatible with. As I am writing, it is 52.0a2. Replace all occurrences with your Thunderbird version number. To find it, go the Help menu and click on About Thunderbird. Save the file and the archive manager will normally asks if you want to update the archive. Answer yes.
5. Double-click on charamel.xpi, it will open a new window with the content of the file. You will see some files: preview.png, license.txt, install.rdf, icon.png, copyright.txt, chrome.manifest and a folder chrome. As before, edit install.rdf and change the version number. Save the file and accept the update of the both archives.
6. Finally, same procedure with the last file silvermelxt.xpi. Edit the install.rdf file and accept the archive update.
7. Now it is time to install it in Thunderbird. Go to the module manager, click on the small cogwheel on the top level and choose install from file. Select the file, and enjoy the theme!
I hope it will help some people. In my opinion, it should be the default Thunderbird theme!
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