By default, Windows 10 adds a transparency effect to the various elements of the user interface including the Taskbar, Action Center, and Start menu. If you’re not a fan of this visual effect, you can turn off the transparency feature in a few simple steps.
Apply a Solid Color to Start Menu, Taskbar, and Action Center
Click on Start > Settings > Personalization > Colors:
Scroll to the bottom of the page (just below the color palette) and toggle the Make Start, taskbar, and action center transparent switch Off: