I am unable to get my custom formatting to override the highlighting on errors

From your other thread, I think that adding the following rules to the bottom of the form's injected CSS should achieve your objective:

.form-line-error {
background: rgba(74,194,194,0.24);
}

.form-line-error
.form-error-message
.form-error-arrow {
border-bottom-color: #51C2C2;
}

Do let us know if you need more help with this.