Add CSS to AMP Legacy theme.

amp-legacy-theme-css.php

<?php
/**
 * Plugin Name:       AMP Legacy Theme CSS
 * Plugin URI:        https://amp-wp.org
 * Description:       Plugin add CSS to AMP Legacy theme.
 * Version:           1
 * Requires at least: 5.6
 * Requires PHP:      7.4
 * Author:            AMP Support
 * Author URI:        https://wpindia.co.in/
 * License:           GPL v2 or later
 * License URI:       https://www.gnu.org/licenses/gpl-2.0.html
 * Update URI:        https://example.com/my-plugin/
 * Text Domain:       amp-legacy-theme-css
 * Domain Path:       /languages
 */
add_action(
	'amp_post_template_css',
	function () {
		// only CSS here please...
		?>
      nav.uk-navbar-container .uk-navbar-left, header#site-header>.uk-container .uk-navbar-container .uk-navbar-right, #offcanvas-nav {
          display: none;
      }
      main#site-main {
          width: 100%;
          display: inline-block;
          position: relative;
          clear: both;
      }
      footer.site-footer {
          position: relative;
          display: inline-block;
          width: 100%;
          height: auto;
      }

      header#site-header>.uk-container {
          padding: 10px;
      }
		<?php
	}
);