How to make the pre-order button look the same as "Add To Cart"?

Go to Design page
Select "Add to cart" button style in the Pre-order button style section
Click OK on the pop-up

Your browser can block the opening of the window. If so you should allow opening the window.
It is necessary to read button styles and update preview.
Check that the preview of the pre-order button has been updated with appearance of add to cart button.
If you have any problems, then you should contact support.
Click Save

Note that if you publish a new shop theme then pre-order button styles remain from the previous theme.