What's the keyboard shortcut to select the non-default button on a pop-up alert? Look at the screenshot. I want to select the non-blue button with a keyboard shortcut. What is it?
Tab key to navigate the focus between buttons, and press the space bar when a button is selected.

The effect is as above image.  When a blue box around a button, press the space bar, the button will be "pressed".

If you found that the focus cannot be set to the button, pen the Keyboard preference, and config it as following: