Open the Settings App.
1
Go to Personalization - Colors.
Set 'Choose your Mode to Custom.
You can then set Windows Mode and App Mode to Light or Dark.
2
In the Settings App, go to Accessibility - Contrast Themes.
If any Contrast Theme is set, disable that, set it to None.
3
Finally if you need to, go to Personalization - Themes, and set that to the Windows Default Themes.