This post is mainly to help others but also to check I have done the right thing.
I have created a button block.
I wanted the buttons to get the default styling I have already set up in appearance / customize /colors /buttons and
appearance / customize /typography / buttons (plus any default button styling from the GP theme)
So for each button, in the block editor under ‘advanced’ I added ‘button’ as a CSS class
Then in the ‘colors’ section I clicked on the ‘background color’, then clicked the ‘clear color’ button. You need to do this for the ‘Normal’ state and the ‘Hover’ state. (You may need to do this for text color as well, but my text color was the same so I left it)
Then in the ‘spacing’ section, clear the values for ‘padding’
Is there an actual ‘clear defaults’ option that would do all that automatically?