Windows 10 has coloured window borders and shadows for each window. Changing the colour of your window borders and disabling the shadow can be done if you wish.
The default border colour on Insider Preview builds of Redstone 5 is grey, so it blends with the shadow, but you can re-enable coloured window borders, too.
How to Choose a Border Color
Windows automatically selects a window colour based on your desktop background in the April 2018 Update and earlier versions of Windows 10.
Window border colours can be customized by going to Settings > Personalization > Colors. Disable “Automatically pick an accent colour from my background” under the “Choose Your Color” section and pick your preferred colour instead.
There are a number of suggested colours here, but you can choose any colour you want by clicking the “Custom Color” option at the bottom of the list.
Redstone 5’s default window border in Windows 10’s stable build will be grey, when it’s released sometime in the fall of 2018. Activate the “Title bars and window borders” option under “Show accent colour on the following surfaces”, and then scroll down to the “Application of accent colour on the following surfaces” section.
As a result of the Sets feature, your title bars will have coloured tabs instead of fully coloured ones.
How to Disable (or Enable) Shadows
Drop shadows are enabled by default for each window in Windows 10. If you prefer, you can disable these shadows. In Windows 8, for example, the window borders were coloured and there were no shadows.
The old Advanced System Settings window contains this setting. You can access it by clicking Start, typing “Advanced system settings” into the search box, and pressing Enter. The Advanced System Settings can also be accessed from Control Panel > System and Security > System.
In the Performance section of the Advanced tab of the System Properties window, click the “Settings” button.
Select the “Custom” option under the Visual Effects list, deselect the “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.