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; other element types don't support changing the font style ('*' is a particular selector that means all elements from the supported ones, so the easiest approach is setting exact types).
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.