I think it's because the taskbar is unaffected by changing your window color and appearance, as shown in the pictures above.

That would make sense, since that taskbar is not a window.

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"Those who talk of blind obedience may appear to know many things, but they do not understand the doctrines of the gospel. There is an obedience that comes from a knowledge of the truth that transcends any external form of control. We are not obedient because we are blind, we are obedient because we can see." -Elder Boyd K. Packer