The steps depend on which version of the app you're using.
If You're Using Magical Preorder (the new version):
You can update the font, color, and size of the preorder elements to match your store’s branding.
In your Shopify admin, go to Online Store > Themes.
Click Customize next to your live theme.
In the theme editor, click App Embeds (top-left corner).
Find Magical Preorder and click the drop-down arrow.
This will open the Style Display Settings. Adjust the font, size, and color.
Click Save in the top-right corner.
Changes will appear instantly on your storefront.
If You're Using Preorder Pro + Back in Stock (the legacy version):
To customize the look and feel of the preorder button:
In the app, click Settings from the left-hand menu.
Click Preorders in the dropdown.
Edit the button text, color, and styling as needed.
Scroll down and click Save in the bottom-right corner.
Your updates will apply across all products using the preorder feature.