Microsoft has added a new theme pack to the Microsoft Store for Windows 10 users. Titled River Delta PREMIUM, the new theme pack contains 10 high-quality 4K wallpapers, the description of which reads as follows:
Seen from above, these waterways take on abstract shapes and colors in this 10-image premium 4k set, free for Windows 10 Themes. These images are to be used as desktop wallpaper only.
After downloading the collection, go to Start > Settings > Personalization > Themes and click on the theme you prefer.
To apply and change to the most appropriate color, based on the current wallpaper, go to Customize > Colors > Automatically select a primary color from the background.