Change button copy on theme preview

This commit makes the dark mode preview feel a little more interactive
and it also pulls some duplicated code into one file so that people like
me aren't stumped when they are looking at the wrong page!

assets/js/dark-mode-preview.js

@@ -0,0 +1,23 @@
+document.addEventListener("DOMContentLoaded", function(){
+
+  const toggleDarkMode = document.querySelector('.js-toggle-dark-mode')
+  const cssFile = document.querySelector('[rel="stylesheet"]')
+  const originalCssRef = cssFile.getAttribute('href')
+  const darkModeCssRef = originalCssRef.replace('just-the-docs.css', 'dark-mode-preview.css')
+  const buttonCopy = ['Return to the light side', 'Preview dark color scheme']
+  const updateButtonText = function(toggleDarkMode) {
+    toggleDarkMode.textContent === buttonCopy[0] ?
+      toggleDarkMode.textContent = buttonCopy[1] :
+      toggleDarkMode.textContent = buttonCopy[0]
+  }
+
+  addEvent(toggleDarkMode, 'click', function(){
+    if (cssFile.getAttribute('href') === originalCssRef) {
+      cssFile.setAttribute('href', darkModeCssRef)
+      updateButtonText(toggleDarkMode)
+    } else {
+      cssFile.setAttribute('href', originalCssRef)
+      updateButtonText(toggleDarkMode)
+    }
+  })
+})