WordPress login/register custom CSS not working?

You can use the register_form hook. Add this to your functions.php:

<?php
add_action( 'register_form', 'custom_css_register_form' );
function custom_css_register_form() {
    ?>
        <style>
            #reg_passmail {
                color: white !important;
            }
        </style>
    <?php
}
?>