Your .tdheader styling only applies to the table cell. Inside the cell there is a label (the HTLML label tag) and this label gets its styling from the .lotusForm td label selector.
You need to override this specific selector or create your own using e.g. the .tdheader label selector .
I can highly recommend overriding the OneUI specific selectors as you then do not need to create your own selectors and add these to the correect XPages controls.