The procedure is indeed not very obvious on first time use. You observed that when you clicked +, the Add Custom Shortcut dialog appears. It contains the fields "Name", "Command" and "Shortcut".
To set the shortcut, first click the "Shortcut" field, i.e., where it says Set Shortcut ....
The dialog now changes to show you a picture of a keyboard and the text Enter the new shortcut. Now press your desired keyboard combination. Once you do so, the keyboard combination will be filled in the "Shortcut" field.
To change the shortcut, click the little left-arrow with a cross in it on the right of the current shortcut combination. Now you can set a different combination the same way as above.
I think the issue here is ergonomic. The behavior is: it expects you to click within the field at which point modifier keys will trigger an input capture.
If this is not what your issue is, then please describe your problem to me more and we'll work it out :)
I found solution. Firstly, I should delete desirable combination if it uses as shortcut in another command.
I found desirable combination on the list on Keyboard settings, then selected this field and pressed Backspace to reset combination from this command.
Now I can set desirable combination to my custom shortcut.
The procedure is indeed not very obvious on first time use. You observed that when you clicked +, the Add Custom Shortcut dialog appears. It contains the fields "Name", "Command" and "Shortcut".
To change the shortcut, click the little left-arrow with a cross in it on the right of the current shortcut combination. Now you can set a different combination the same way as above.
I think the issue here is ergonomic. The behavior is: it expects you to click within the field at which point modifier keys will trigger an input capture.
If this is not what your issue is, then please describe your problem to me more and we'll work it out :)