Hi

Here is an  app which is able to design your templates for quotation and invoice.

https://apps.odoo.com/apps/modules/16.0/base_advanced_report_templates/

Hope it helps

Hi 

You can customize it by 

Go to settings > Enable the Debug Mode > Technical > Reports > Choose the Report Template that needs to modify > Qweb views

By clicking on the edit button you can change the current code.

Regards