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
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.