Add eventlistener for change of day/night mode

And add a static js folder for js-files apart from the asset pipeline.

priv/static/js/dark-mode.js

@@ -0,0 +1,10 @@
+function set_day_night_mode(){
+  // 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.documentElement.classList.add('dark')
+  } else {
+    document.documentElement.classList.remove('dark')
+  }
+}
+set_day_night_mode()
+matchMedia('(prefers-color-scheme: dark)').addEventListener("change", set_day_night_mode)