I have recently been in contact with the developer who made the ever-so-popular FT DeepDark and TT DeepDark themes for Firefox and ThunderBird and he is says that he is going to see if he can make a GTK theme.
"I like to create skins/themes and when I will have the possibility I will try to see if I can make a GTK theme"
but he wants people to answer his questions "In case I will need some help when I will try to make a GTK theme"
FT DeepDark currently has an average of 200,903 daily users, 13,364 of which run Linux with a total of 5,466,251 downloadshttps://addons.mozilla.org/en-US/firefox/addon/ft-deepdark/?src=search
TT DeepDark has and average of 43,318 daily users, 6,929 are Linux users with 799,609 total downloadshttps://addons.mozilla.org/en-US/thunderbird/addon/tt-deepdark/?src=search
Needless to say, these are very popular themes