I believe Error Text and Error Background are for the Error label and not the Input field. That said, I do see that they have no effect on the error label and I suggest that you raise a Support Request to report this.
The approach suggested didn't work as expected on my local Pega 7.1.9 and below is the response from GCS support engineer.
After thorough investigation of the reported behaviour, our Product Specialist has confirmed that the reported behaviour is a BUG in the product, and fixing it within this ML version will take unprecedented changes.
Therefore, we have registered it as a bug (bug ID: BUG-230504) for the same. Currently there is no alternate solution/local change to get around this discrepancy, and the scrum team has considered to fix it (probably) in Pega 7.2.1.
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