Create Twig Extension

Create file custom_base\src\TwigExtension\ActiveThemeUrlExtension.php:

<?php

namespace Drupal\custom_base\TwigExtension;

use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Extension\ThemeHandlerInterface;
use Twig\Extension\AbstractExtension;
use Twig\TwigFunction;

/**
 * Provides a Twig extension to retrieve the active theme absolute url.
 */
class ActiveThemeUrlExtension extends AbstractExtension {

  /**
   * Theme Handler service.
   *
   * @var \Drupal\Core\Extension\ThemeHandlerInterface
   */
  protected $themeHandler;

  /**
   * The config factory.
   *
   * @var \Drupal\Core\Config\ConfigFactoryInterface
   */
  protected $configFactory;

  /**
   * Constructs a new ActiveThemeUrlExtension object.
   *
   * @param \Drupal\Core\Extension\ThemeHandlerInterface $themeHandler
   *   The theme handler.
   * @param \Drupal\Core\Config\ConfigFactoryInterface $configFactory
   *   The config factory.
   */
  public function __construct(
    ThemeHandlerInterface $themeHandler,
    ConfigFactoryInterface $configFactory
  ) {
    $this->themeHandler = $themeHandler;
    $this->configFactory = $configFactory;
  }

  /**
   * {@inheritdoc}
   */
  public function getFunctions(): array {
    return [
      new TwigFunction('absolute_theme_url', [
        $this,
        'getActiveThemeAbsoluteUrl',
      ]),
    ];
  }

  /**
   * Retrieves the active theme absolute url.
   *
   * @return string
   *   The absolute url to the active theme.
   */
  public function getActiveThemeAbsoluteUrl(): string {
    global $base_url;

    $config = $this->configFactory->get('system.theme');
    $defaultThemeName = $config->get('default');
    $theme = $this->themeHandler->getTheme($defaultThemeName);
    $theme_path = $theme->getPath();

    return $base_url . '/' . $theme_path;
  }

}

Add to custom_base.services.yml:

af2_base.twig_extension:
  class: Drupal\af2_base\TwigExtension\ActiveThemeUrlExtension
  arguments: ['@theme_handler', '@config.factory']
  tags:
    - { name: twig.extension }

Now in twig templates available {{ absolute_theme_url() }}