Basic setup, launch config and layout

app/javascript/controllers/index.js

@@ -6,3 +6,6 @@ import { application } from "./application"
 
 import HelloController from "./hello_controller"
 application.register("hello", HelloController)
+
+import SetThemeController from "./set_theme_controller"
+application.register("set-theme", SetThemeController)

app/javascript/controllers/set_theme_controller.js

@@ -0,0 +1,18 @@
+import { Controller } from "@hotwired/stimulus"
+import Cookie from "../lib/cookies"
+
+// Connects to data-controller="set-theme"
+export default class extends Controller {
+  COOKIE_NAME = "modeTheme";
+
+  connect() {
+    const cookieValue = Cookie.get(this.COOKIE_NAME);
+    console.log("cookieValue", cookieValue);
+    if(cookieValue) {
+      return true;
+    }
+    const darkMode = window.matchMedia("(prefers-color-scheme:dark)").matches ? "dark" : "light";
+    Cookie.set(this.COOKIE_NAME, darkMode);
+    window.document.getElementsByTagName("html")[0].setAttribute("data-bs-theme", darkMode)
+  }
+}