chore: move shikiji workaround (vuejs#2666)

Author: antfu

.vitepress/config.ts

@@ -711,6 +711,7 @@ export default defineConfigWithTheme<ThemeConfig>({
   },
 
   markdown: {
+    theme: 'github-dark',
     config(md) {
       md.use(headerPlugin)
       // .use(textAdPlugin)

.vitepress/theme/styles/index.css

@@ -4,8 +4,3 @@
 @import "./inline-demo.css";
 @import "./utilities.css";
 @import "./style-guide.css";
-
-/* vitepress rc.31 migrated to shijiki and need this to apply code styles */
-.vp-code span {
-  color: var(--shiki-dark, inherit);
-}