Refactored and improved dark-mode implementation

frontend/src/app/services/cookie.service.ts

@@ -0,0 +1,34 @@
+import {Injectable} from '@angular/core';
+
+@Injectable({
+  providedIn: 'root'
+})
+export class CookieService {
+
+  constructor() {
+  }
+
+  setCookie(name: string, value: any): void {
+    document.cookie = `${name}=${value.toString()};`;
+  }
+
+  getCookie(name: string): string | null {
+    let end;
+    const dc = document.cookie;
+    const prefix = name + "=";
+    let begin = dc.indexOf("; " + prefix);
+
+    if (begin == -1) {
+      begin = dc.indexOf(prefix);
+      if (begin != 0) return null;
+    } else {
+      begin += 2;
+      end = document.cookie.indexOf(";", begin);
+      if (end == -1) {
+        end = dc.length;
+      }
+    }
+
+    return decodeURI(dc.substring(begin + prefix.length, end));
+  }
+}