How to make custom CSS for search in WP?

ref https://www.sitepoint.com/community/t/how-to-make-custom-css-for-search-in-wp/357614/9

<?php get_search_form(); ?>

if you check the DOM, you can see the code generated by WP:
<form role="search" method="get" class="search-form" action="">
    <label>
        <span class="screen-reader-text">Search by:</span>
        <input type="search" class="search-field" placeholder="Search" value="" name="s" />
    </label>
    <input type="submit" class="search-submit" value="Search" />
</form>

<form action="/" method="get">
    <label for="search">Search in <?php echo home_url( '/' ); ?></label>
    <input type="text" name="s" id="search" value="<?php the_search_query(); ?>" />
    <input type="image" alt="Search" src="<?php bloginfo( 'template_url' ); ?>/images/search.png" />
</form>

หรือ https://5balloons.info/customise-search-widget-wordpress/