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)
+  }
+}

app/javascript/lib/cookies.js

@@ -0,0 +1,23 @@
+export default class Cookies {
+  static set(cname, cvalue, exdays = 365) {
+    const d = new Date();
+    d.setTime(d.getTime() + (exdays * 24 * 60 * 60 * 1000));
+    let expires = "expires="+d.toUTCString();
+    document.cookie = cname + "=" + cvalue + ";" + expires + ";path=/";
+  }
+  
+  static get(cname) {
+    let name = cname + "=";
+    let ca = document.cookie.split(';');
+    for(let i = 0; i < ca.length; i++) {
+      let c = ca[i];
+      while (c.charAt(0) == ' ') {
+        c = c.substring(1);
+      }
+      if (c.indexOf(name) == 0) {
+        return c.substring(name.length, c.length);
+      }
+    }
+    return "";
+  }
+}