Rated 5 out of 5 stars

I patched it to work with TB 60+ (up to 111)... works fine for me. Also note that you have to change the quote color on replies via CSS. See my gist for download and further instructions for dark TB:

https://gist.github.com/novalis111/4451dbe2e63c72fb4d8f35f7ac8c45c2#thunderbird-dark