Mac keyboard shortcut to press the replace button

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:

Create a plain text file with this in it (in TextEdit, use Format > Make Plain Text):

{
    "~d" = ("insertText:", "ð");
}

Call it DefaultKeyBinding.dict (make sure to uncheck “When no extension is provided, use .txt”), and save it in ~/Library/KeyBindings/. (You can press Command ⌘+Shift ⇧+G and paste that path.)

The "~d" means Option-D, and the ("insertText:", "ð") means it will insert a ð character instead of ∂.

You will have to restart your applications in order for this setting to take effect.