1
Open the Settings App.
Go to Accessibility - Contrast Themes.
Disable that contrast theme.
Then go to Personalization - Themes and set the Windows Default Theme.
Restart (not shut down) your PC.
Then go back in and set your theme to the Night Sky contrast theme to test if that theme is then set correctly.
2
If not:
Click your Start Button, type winver and press Enter
On the resulting dialog, what version of Windows 10 are you running?