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Here’s how to achieve that effect.
Open Terminal, then copy/paste this and press Return: defaults write NSGlobalDomain AppleInterfaceStyle Dark
Paste or type killall Dock and press Return. The Dock will relaunch in its dark mode.
Copy/paste this and press Return: defaults remove NSGlobalDomain AppleInterfaceStyle
The first step sets dark mode, step two restarts the Dock to switch it to dark mode, and step three turns off dark mode—but the Dock won’t notice, and will remain in its dark state (until it’s next restarted, which isn’t often). Because the Command-Tab switcher is associated with the Dock, it will also be dark.