Rated 4 out of 5 stars

Great for supporting Thunderbird latest version instead of the original plugin.

But, on v3.2.4, it changed style CSS setting to use its own one.
Styles for the original ver. is not inherited. Gee...

And, on this ver., syntax highlighting is not working, does it?

Now it looks having several problems on CSS settings.
Unfortunately, unwelcome update...

This review is for a previous version of the add-on (3.2.4). 

Yes, syntax highlighting is broken in 3.2.4. Version 3.2.5 will have fixes for that and several other issues.
This is a new addon with its own ID, so it it not able to access the extension storage from the original Markdown Here.
If you modify the CSS to suit your taste in settings, your changes will be kept on future updates. If that's not the case, it's a bug.