Hi, if you mean Windows 11 desktop themes, you can add more city backgrounds this way:
Open Settings >> Personalization >> Themes >> Browse themes.
Choose a theme in Microsoft Store and select Get.
Once installed, return to Settings >> Personalization >> Themes and click the new theme to apply it.
For example, Microsoft offers the free Panoramic Cityscapes theme here : https://apps.microsoft.com/detail/9n5mtn76v9nv