Simplify access to html element in dark mode snippet (#721)

https://developer.mozilla.org/en-US/docs/Web/API/Document/documentElement

Author: philippbosch

src/pages/docs/dark-mode.mdx

@@ -115,9 +115,9 @@ Here's a simple example of how you can support light mode, dark mode, as well as
 ```js
 // On page load or when changing themes, best to add inline in `head` to avoid FOUC
 if (localStorage.theme === 'dark' || (!('theme' in localStorage) && window.matchMedia('(prefers-color-scheme: dark)').matches)) {
-  document.querySelector('html').classList.add('dark')
+  document.documentElement.classList.add('dark')
 } else {
-  document.querySelector('html').classList.remove('dark')
+  document.documentElement.classList.remove('dark')
 }
 
 // Whenever the user explicitly chooses light mode