You cannot customise the Windows 10 Mobile without apps as Microsoft hasn’t really improved the customization feature in the OS. While on other hand, there are many ways and apps to customize Windows 10 PCs and Tablets.
With Dynamic Theme, you can customize Windows 10 Mobile by changing the start and lock screens background. The app will change the lock screen picture with daily Bing or Windows Spotlight pictures. Furthermore, you can also use personal pictures as lock screen or start screen background.
The app comes with support for multiple modes, you can put the static image or slideshow. Furthermore, it also lets you view recent daily Bing and Windows Spotlight images. The Dynamic Theme will send a notification when a new daily Bing or Windows Spotlight image is published.
The best part is that it automatically synchronizes the settings and customizations between your Windows 10 devices, only when the devices are on the same Microsoft Account. Dynamic Theme overall does a good job and it is also available for PCs. You can download the app from the below Windows Store link.