Add eventlistener for change of day/night mode

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

assets/js/dark-mode-widget.js

@@ -0,0 +1,36 @@
+let DarkModeHook = {
+
+  mounted() {
+    this.button = this.el.querySelector("button")
+    this.button.addEventListener("click", this.show_selector.bind(this))
+    this.selector = this.el.querySelector("ul")
+    var lis = this.el.querySelectorAll("li")
+    lis[0].addEventListener("click", this.switch_to_day_mode.bind(this))
+    lis[1].addEventListener("click", this.switch_to_night_mode.bind(this))
+    lis[2].addEventListener("click", this.switch_to_system_mode.bind(this))
+  },
+  show_selector(){
+    this.selector.classList.remove("hidden")
+  },
+  switch_to_day_mode() {
+    document.documentElement.classList.remove('dark')
+    localStorage.theme = 'light'
+    this.selector.classList.add("hidden")
+  },
+  switch_to_night_mode() {
+    document.documentElement.classList.add('dark')
+    localStorage.theme = 'dark'
+    this.selector.classList.add("hidden")
+  },
+  switch_to_system_mode() {
+    if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
+      document.documentElement.classList.add('dark')
+    } else {
+      document.documentElement.classList.remove('dark')
+    }
+    localStorage.removeItem('theme')
+    this.selector.classList.add("hidden")
+  },
+}
+
+export {DarkModeHook}