Thanks for your help, Vad_M, really.
Not at all, Sean!
So, can you help me to post all other css files with the small instructions (if it's necessary, of course)? It will be a few thousand lines of code for Windows 7 and Windows 10 and for the different versions of Firefox, Thunderbird and Tor (ff-52+ win7, ff-52+ win10, ff-60+ win7, ff-60+ win10, th-52+ win7 and tor-8+ win7, tor-8+ win10).
It seems to me that any problems with Mozilla products skinning will be solved...