The default styling for header & footer can't be different from the default styling for the main document.
If your goal is to set default styling, which should be applied over the main document, the header and the footer, then the correct way to do this is to apply the styling over the main document style. Here is example on how to do it:
.radRichTextBox.Document.Style.SpanProperties.FontSize = Unit.PointToDip(26);
.radRichTextBox.Document.Style.SpanProperties.ForeColor = Colors.Red;
If you would like to know more about the styles in RadRichTextBox I can recommend you the styles
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