/usr/share/themes/Default/gtk-3.0/gtk-keys.css is in libgtk-3-common 3.4.1-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | /*
* Default keybinding set. Empty because it is implemented inline in the code.
*/
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