Hello,
You can disable the “Default Thank you page” from Theme Settings > Shop and create an HTML block with your structure and content. Then add the ID shortcode of the block in the “Extra content of Thank you page”
If you want to use the default thank you page and want to customization (like colors, spacing, layout), you need to use the custom css code. If you need custom css code, share the page URL so I can further check on your site and give you a possible css code.
Best Regards,