Outlook not showing the toggle dark mode on/off button on the message window
Solution 1:
The accepted answer doesn't seem to be valid anymore.
For me the link provided by the OP has the right information.
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Reading a message you received
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When reading a message, you can turn dark mode on or off by using the icon at the top of the message window.
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composing a message
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When composing a message, you can turn dark mode on or off from the Message toolbar by selecting the dark mode icon to change the message window to white or black.
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