refactor(theme): update build template

packages/theme/templates/build.html

@@ -4,17 +4,6 @@
     <meta charset="utf-8">
     <meta name="viewport" content="width=device-width,initial-scale=1">
     <meta name="generator" content="VuePress {{ version }}">
-    <style>
-      :root {
-        --c-bg: #fff;
-      }
-      html.dark {
-        --c-bg: #22272e;
-      }
-      html, body {
-        background-color: var(--c-bg);
-      }
-    </style>
     <script>
       const userMode = localStorage.getItem('vuepress-color-scheme');
 			const systemDarkMode = window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches;