Customizing Top Black Bar in Invoice and Packaging Slip

Hello @cocosugar

You can use the code snippet below:

add_action( 'wpo_wcpdf_custom_styles', 'wpo_wcpdf_custom_styles', 10, 2 );
function wpo_wcpdf_custom_styles ( $document_type, $document ) {
    ?>
    .order-details thead th {
        color: black; /* this is the text color of the header row */
        background-color: white;
        border: 0 0 2pt 0;
        border-color: black;
    }
    <?php
}

If you haven’t worked with actions/filters before please refer this documentation page: How to use filters