Widget CSS Classes: Define your own classes through filters.

wcssc-class-filters.php

<?php

/**
 * Plugin hooks:
 * widget_css_classes_first
 * widget_css_classes_last
 * widget_css_classes_even
 * widget_css_classes_odd
 * widget_css_classes_number
 *
 * Passes 1 parameter: the current class
 */
add_filter( 'widget_css_classes_first', 'custom_widget_css_classes_first' );
function custom_widget_css_classes_first( $class ) {
  return 'widget-first'; // You can return any valid CSS class.
}