related_post_element_css_$id

You can hook custom CSS for each element

add_action('related_post_element_css_custom','related_post_element_css_custom_20200126');

function related_post_element_css_custom_20200126($elementData) {

    //$option_id_value = isset($elementData['option_id']) ? $elementData['option_id'] : '';
    $margin = isset($elementData['margin']) ? $elementData['margin'] : '10px';

    ?>
    .related-post .post-list .item .post_title{
    margin: <?php echo $margin; ?>
    }
    <?php

}