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#11
Specifically that option is grayed out even under 'Custom' when you choose 'Light' for default Windows mode.
You must choose 'Dark' for 'default Windows mode'.
and either 'Light' or 'Dark' for 'default app mode'.
Then the 'accent color' for Start, taskbar and action center will be available
Worth noting: The 'Dark' for 'default Windows mode' isn't as dark as you think with 'Light' set for 'default app mode'
You can also change the color of inactive windows via this tutorial:
How to Change Color of Inactive Title Bar in Windows 10