Hello,
On https://www.phpdocx.com/api-documentation/docxcustomizer/customize-docx-Word-documents-PHP you can read the supported styles for each element. The font style is supported in paragraph and run elements.
Also, note this method includes the target option:
target string document (default), style, lastSection, header, footer.
By default, the target is document, but you can also set the following targets: style, header, footer.
Please check the samples included in the package (examples/DOCXCustimizer folder), that illustrate how to use DOCXCustomizer.
Regards.