While the BACKGROUND of your certificates is not customizable without development, you can remove the certificate badge and change all of the text content and logo that overlay the background.

You can change the background to two designs (modern or classic) and three colors (purple, gold and blue) via the Options tab of your Certification Survey:

Via the Reporting Menu (in Developer Mode), you can find the Certifications report, and from there you can click the QWeb Views button to find the certification_report_view_modern and certification_report_view_classic views to change the content overlaying the background.

Inherit and override the relevant view to make your changes. Don't edit the base View or these changes will not survive a database upgrade.

Before:

After:

check my module if it maybe help https://apps.odoo.com/apps/modules/14.0/survey_upload_custom_certificate/

So far, we CAN NOT use a custom design/template for the certificate. 
Let's make a wish for it. :)