QB Issue Resolution:

you can not customize any from in QBO other than the invoice

gear>feedback and report/suggest it to the developers

Resolution for Issue 'How do you customize the look of a credit note, like you do the sales invoice on Quickbooks Online Plus?' available: Yes (Solved).
Source: Intuit Community forum.

How do you customize the look of a credit note, like you do the sales invoice on Quickbooks Online Plus?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for Invoice
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Error Recovery for Invoice [Canada]
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