extract PatternPreview component styles.

src/page-components/pattern-library/PatternDetail.tsx

@@ -15,7 +15,13 @@ const EmptyPromption: FC = () => {
 };
 
 const Detail: FC<{ pattern: Pattern }> = ({ pattern }) => {
-  return <div className={styles.pattern_detail}></div>;
+  return (
+    <div className={styles.pattern_detail}>
+      <div></div>
+      <div></div>
+      <div></div>
+    </div>
+  );
 };
 
 const PatternDetail: FC = () => {