feat: NCS-63 User account implementation (#2)

User Profile info, no game before login/register, menu bar

---------

Co-authored-by: Lala, Shahd <Shahd.Lala@sybit.de>
Co-authored-by: shahdlala66 <shahd.lala66@gmail.com>
Reviewed-on: #2

src/app/services/theme.service.ts

@@ -0,0 +1,34 @@
+import { Injectable } from '@angular/core';
+import { BehaviorSubject } from 'rxjs';
+
+@Injectable({ providedIn: 'root' })
+export class ThemeService {
+    private readonly darkModeSubject = new BehaviorSubject<boolean>(false);
+
+    readonly darkMode$ = this.darkModeSubject.asObservable();
+
+    initTheme(): void {
+        const savedTheme = localStorage.getItem('theme');
+        this.applyDarkMode(savedTheme === 'dark');
+    }
+
+    toggleTheme(): void {
+        this.applyDarkMode(!this.darkModeSubject.value);
+    }
+
+    setDarkMode(isDarkMode: boolean): void {
+        this.applyDarkMode(isDarkMode);
+    }
+
+    private applyDarkMode(isDarkMode: boolean): void {
+        if (isDarkMode) {
+            document.documentElement.setAttribute('data-theme', 'dark');
+            localStorage.setItem('theme', 'dark');
+        } else {
+            document.documentElement.removeAttribute('data-theme');
+            localStorage.removeItem('theme');
+        }
+
+        this.darkModeSubject.next(isDarkMode);
+    }
+}