xorg - GTK 3.22 migration of older gtk.css -> Black Margin on elementaryOS apps - elementary OS Stack Exchange
gnome - how do I change the CSS for a GTK application? - Ask Ubuntu
I modified the gtk style by adding a gtk.css. Now the border is missing in nautilus & settings window but terminal has a border. What am i doing wrong? : r/linux4noobs
Tutorial for making GTK3 themes
Enabling the GTK Inspector - to find CSS values for theme styles - Tips & Tricks - Ubuntu MATE Community