After installing the Windows 10 Feature Update 1903 or higher, you may have noticed that the accent color checkbox Start, taskbar, and action center is grayed out. The recent Windows 10 versions come with a new Light Theme (“Light” color mode) that displays the Taskbar, Start menu, action center, notification area pop-ups, etc., in light grey color.
When the light color mode is enabled for Windows, the Taskbar, Start menu, and the action center can’t be colorized, which is why those settings are dimmed out. This is by design. You’ll be able to show an accent color for window title bars and borders, though.

Here is how to enable the dark color mode for Taskbar, Start Menu, and the Action Center. The current version of Windows 10 is 20H2 as of this post, and the instructions apply to version 20H2 as well.
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