On Windows 10, each window has a colored window border and a shadow. You can change the color of your window borders and even disable the shadow, if you like.
Insider Preview builds of Redstone 5 change the default border color to gray so it blends in with the shadow, but you can re-enable colored window borders on Redstone 5, too.
How to Choose a Border Color
In the April 2018 Update and earlier versions of Windows 10, Windows automatically chooses a window color that matches your desktop background.
To choose a custom window border color, head to Settings > Personalization > Colors. Under the “Choose Your Color” section, disable the “Automatically pick an accent color from my background” option and choose your preferred color, instead.
Windows provides a number of suggested colors here, but you can click the “Custom Color” option at the bottom of the list to choose any color you like.
Windows 10’s Redstone 5 update, which will be released to the stable Windows build sometime in Fall, 2018, uses a gray window border by default. To apply your accent color to your window borders, scroll down to the “Show accent color on the following surfaces” section, and then enable the “Title bars and window borders” option.
How to Disable (or Enable) Shadows
Windows 10 enables drop shadows for each window by default. However, you can turn these shadows off if you prefer. Windows 8 didn’t use shadows and had a clean, flat look with colored window borders, for example.
This setting is available in the old Advanced System Settings window. To open it, hit Start, type “Advanced system settings” into the search box, and then press Enter. You can also head to Control Panel > System and Security > System > Advanced System Settings to launch it.
On the Advanced tab of the System Properties window, click the “Settings” button in the Performance section.
In the Visual Effects list, select the “Custom” option, disable the “Shadow shadows under windows” option, and then click the “OK” button.
Window shadows will instantly vanish. You can return here if you ever want to re-enable them.