Customizer Way
For All Galleries: To change the default gallery style for all your galleries, navigate to any gallery on your site and click Customize > Content and Design.
Make sure you select All Galleries in the top section of your Customizer window, then click the wrench icon next to Gallery.
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Change your Default Style by using the dropdown menu to select your preference from the list.
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While you’re there, you can also tweak the global settings for your preferred style. Select the style name from the ordered list below the dropdown section to open its settings panel.
There are screen shots on the help page; it may easier to understand there.
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--- Denise
Musings & ramblings at https://denisegoldberg.blogspot.com