In the build released today, Dark Theme looks much better than we think. If you still look forward to the Dark Theme, perhaps what Microsoft does is not disappointing at all.
This is the real Dark Theme appearance of File Explorer (click on the image to see the full size)
First, make sure you have updated to the latest OS, then go to Settings > Personalization > Colors.
Under this section there will be Choose your default app mode with 2 options, Light and Dark. To turn on the Dark Theme, simply select Dark as shown in the screen below.
The setting section selects the color mode
When Dark is selected , Windows will turn on Dark Theme for all applications, supported interfaces, including File Explorer.
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