Fix background and fix PageElement system

src/page/main/main.ts

@@ -0,0 +1,10 @@
+import { PageElement } from '../page-element';
+import { generate } from './main.html';
+import { createElement } from '../../helper/create-element';
+
+export class Main extends PageElement {
+  constructor(...children: Array<PageElement>) {
+    super(createElement(generate()), children);
+    children.forEach(c => this.attachElement(c));
+  }
+}