Hello,
On https://www.phpdocx.com/api-documentation/docxcustomizer/customize-docx-Word-documents-PHP you can read all supported styles in customizeWordContent.
A table-cell element supports the following style to set border styles:
borderColor string Hexadecimal color value: 'FF0000', '000000'... This value can be overridden for each side with the use of 'borderTopColor', 'borderRightColor', 'borderBottomColor' and 'borderLeftColor' properties.
Regards.