Add hook_preprocess_page to not strip Drupal CSS.

starshot_demo/starshot_demo.theme

+<?php
+
+/**
+ * @file
+ * Functions to support theming in the Starshot Demo theme.
+ */
+
+/**
+ * Implements hook_preprocess_page().
+ */
+function starshot_demo_preprocess_page(array &$variables): void {
+
+  // Do not strip out Drupal core styles.
+  $variables['modifier_class'] = FALSE;
+
+}