Select the Start 
button, then select Settings > Personalization to choose a picture worthy of gracing your desktop background, and to change the accent color for Start, the taskbar, and other items. The preview window gives you a sneak peek of your changes as you make them.
In Background, select a picture or solid color, or create a slideshow of pictures.
In Colors, let Windows pull an accent color from your background, or choose your own color adventure.
After you've selected an accent color, scroll down to decide where you want to see the color show up, and whether it looks better in a dark or light setting.
The above screen shows the Dark setting.
Notes:
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Get Windows 10 themes in the Microsoft Store
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Personalize your Windows 10 device with themes—a combination of pictures, colors, and sounds—from the Microsoft Store.