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Yes, in the background window in this screenshot https://blog.thunderbird.net/files/2020/07/Dark-Mode-1.png you can see manual Light and Dark options as well as Auto.

Those are the operating system's settings in Mac OS. It's not a Thunderbird dialogue (unfortunately).

Have it installed, such an option isn't available to me, in Win10

According to https://support.mozilla.org/en-US/questions/1291071 it should be under Addons -> Themes, at least in previous versions? But it was kinda buggy as of last month.