The themes for GTK 3.0 are packaged in a gtk.gresource file that contains .css and other files for the theme. I do not understand why, but I have figured out how to extract the files on Ubuntu 19.04.
The gresource utility allows the listing of files in a gtk.gresource file.
jason@jason-Virtual-Machine:~/.themes/Yaru/gtk-3.0$ gresource list gtk.gresource |
This is a sample of the output this program will give when listing files for the Yaru theme.
/com/ubuntu/themes/Yaru/3.0/assets/slider-vert-scale-has-marks-below-backdrop@2.png /com/ubuntu/themes/Yaru/3.0/assets/slider-vert-scale-has-marks-below-hover.png /com/ubuntu/themes/Yaru/3.0/assets/slider-vert-scale-has-marks-below-hover@2.png /com/ubuntu/themes/Yaru/3.0/assets/slider-vert-scale-has-marks-below-insensitive.png /com/ubuntu/themes/Yaru/3.0/assets/slider-vert-scale-has-marks-below-insensitive@2.png /com/ubuntu/themes/Yaru/3.0/assets/slider-vert-scale-has-marks-below.png /com/ubuntu/themes/Yaru/3.0/assets/slider-vert-scale-has-marks-below@2.png /com/ubuntu/themes/Yaru/3.0/assets/text-select-end-active.png /com/ubuntu/themes/Yaru/3.0/assets/text-select-end-active@2.png /com/ubuntu/themes/Yaru/3.0/assets/text-select-end-hover.png /com/ubuntu/themes/Yaru/3.0/assets/text-select-end-hover@2.png /com/ubuntu/themes/Yaru/3.0/assets/text-select-end.png /com/ubuntu/themes/Yaru/3.0/assets/text-select-end@2.png /com/ubuntu/themes/Yaru/3.0/assets/text-select-start-active.png /com/ubuntu/themes/Yaru/3.0/assets/text-select-start-active@2.png /com/ubuntu/themes/Yaru/3.0/assets/text-select-start-hover.png /com/ubuntu/themes/Yaru/3.0/assets/text-select-start-hover@2.png /com/ubuntu/themes/Yaru/3.0/assets/text-select-start.png /com/ubuntu/themes/Yaru/3.0/assets/text-select-start@2.png /com/ubuntu/themes/Yaru/3.0/gtk-dark.css /com/ubuntu/themes/Yaru/3.0/gtk.css |
Then once the file paths are known, extract them with this command.
gresource extract gtk.gresource /com/ubuntu/themes/Yaru/3.0/gtk.css > gtk.css |
This program extracts to STDOUT, so it must be redirected to a filename. But it does work very well. And I can edit the theme now and that is what counts. This is a very simple tip once you know how to perform the operation. Just copy the themes to ~/.themes before editing them. Then you may use Gnome Tweak to select the theme after editing to try it out.