"Gtk Warning: Theme parsing error" for no-longer-installed extension

Likely the file ~/.config/gtk-3.0/gtk.css contains @import url(filename) statements, referring to non-existent files in .../extensions/no-title-bar.

Remove the erroneous import statements from the file gtk.css.

If no-title-bar is still registered as installed:

$ gsettings get org.gnome.shell enabled-extensions | grep --color no-title-bar && echo installed

You can remove its entry from the array org.gnome.shell.enabled-extensions in dconf-editor (apt install dconf-editor) by browsing to org, then gnome, then shell, etc.

There also might remain its entry in org.gnome.shell.extensions. Remove all its sub-keys and the entry org.gnome.shell.extensions.no-title-bar will be removed as well.

Github Forum post

OOO:

You can check enabled extensions with:

gsettings get org.gnome.shell enabled-extensions

If no-title-bar extension is listed, remove it from the array with gsettings or dconf-editor.

enbQao:

...there was still an entry there for that extension. I removed that entry and all other entries I could find... this did not solve the warning. It still shows.

enbQao:

The warning was due to a import statement from ~/.config/gtk-3.0/gtk.css Removing these statements and the warning will not show up anymore.

on 20.04, after upgrading from 18.04

this problem re-surfaced for me recently

my cause & solution:

• there was a snap package installed, gtk-themes-common (I think), which contained Yaru theme files
• it clashed with the system installed gtk-theme Yaru
• removal of the snap solved my problems