There's no focus indicator on buttons when using Breeze/Breeze-Dark GTK2 or GTK3 themes, making using the keyboard to navigate a bit difficult. Focus indicators are visible when using Adwaita or Arc, and the focus highlight for the QtStyle works, so it's specific to the gtk styles.
Steps to Reproduce:
1. Open gtk3-demos, and then run the button boxes demo.
2. Use the [Tab] key to move around the buttons.
No focus indicator to see what button will be pressed when pressing [Space] or [Enter]
Expect to see some kind of focus indicator (An underline or highlight would stylistically match the breeze qtstyle) to see what button will be pressed when pressing [Space] or [Enter].
This was fixed in https://cgit.kde.org/breeze-gtk.git/commit/?id=22637c221ee34694ddb18f7c0ec177a0da3b3dce