A Windows 10 Themes is a short package in Windows 10 that contains one or more background images, accent colour, and, in some cases, music, and you may choose from hundreds of them in the Microsoft Store.
Follow these instructions to download and install a Windows 10 Themes :
Click windows key
Click on setting
Click on Personalization
Choose the theme you like
If there is no stock theme you prefer, you may click on “Get more themes in Microsoft Store”