Hi Eoehen,
Thank you for reaching out to Syncfusion support.
We have checked the reported issue on our end. The `HtmlHeader` and `HtmlFooter` properties only support inline styles. We are unable to set external CSS properties or link external CSS files to apply styles for the header and footer. Please refer the below documentation.
Background color missing issue in HTML Header and Footer
However, we have a workaround solution to set the Custom CSS for the PdfHeader and PdfFooter properties. In this method, we can assign the PdfPageTemplateElement as the Header and Footer. We have attached a sample for your reference.
Header and footer
Sample: https://www.syncfusion.com/downloads/support/directtrac/general/ze/Custom_CSS_Header_Footer-1201616891
Kindly try the provided sample on your end and get back to us if you need further assistance.
Regards,
Karmegam