Once you have some custom cover styles created, you can assign them to be a default on your brands. This way, anytime a new gallery is created with that brand, the custom cover style is automatically applied.

Go to Settings > Branding > Brands and go into your brand. Click the "Gallery Cover" section and choose your default Gallery Cover.

Select the default cover style for that brand, and it will be automatically applied the next time you create a Gallery using this brand!

To use your new custom cover style in a gallery, go into the Settings section of your gallery and click in the Design section of the menu. On the left-hand side, you can cycle through your Gallery Covers. You can either click "Add New" to create a new one, or hover over an existing cover style and click "Edit" to update it.