Clear frontend caches only if theme files changed.

drush.services.yml

services:
  my_module.commands:
    class: \Drupal\my_module\Commands\MyModuleDrushCommands
    tags:
      - { name: drush.command }

my_module.module

/**
 * Lighter version of drupal_flush_all_caches() function.
 *
 * This function clears frontend caches (css, js and twig) only.
 *
 * @see drupal_flush_all_caches()
 */
function my_module_flush_frontend_caches() {
  // Frontend data can remain in these bins, so we need to empty them as well.
  $bins_to_clear = [
    'render',
    'dynamic_page_cache',
    'page'
  ];

  foreach (Cache::getBins() as $key => $cache_backend) {
    if (in_array($key, $bins_to_clear)) {
      $cache_backend->deleteAll();
    }
  }
  
  // Flush asset file caches.
  \Drupal::service('asset.css.collection_optimizer')->deleteAll();
  \Drupal::service('asset.js.collection_optimizer')->deleteAll();
  _drupal_flush_css_js();

  // Wipe the Twig PHP Storage cache.
  \Drupal::service('twig')->invalidate();

}

MyModuleDrushCommands.php

<?php
// Place this file into my_module/src/Commands/MyModuleDrushCommands.php

namespace Drupal\my_module\Commands;
use Drush\Commands\DrushCommands;
/**
 * Class MyModuleDrushCommands.
 */
class MyModuleDrushCommands extends DrushCommands {
  /**
   * Clears only frontend caches.
   *
   * This is the lighter version of drush cr. It only clears frontend caches,
   * so it make sense to use it instead of drush cr if only .css, .js, .twig
   * or .theme files were changed.
   *
   * @command cr:frontend
   *
   * @aliases crf, cache-rebuild-frontend
   */
  public function cacheRebuildFrontend() {
    my_module_flush_frontend_caches();
  }
}