Is this problem in all apps in Windows or just in the Office apps?
If the problem is in all apps.
1
Open the Settings App.
Go to Accessibility - Contrast Themes.
If there is any Contrast Theme set, disable it.
2
Then, go to Personalization - Colors
In there, you have a few options.
If 'Choose your Mode is set to Dark, set it to Light
Or you can set that to Custom.
Then set Windows Mode to Dark and App Mode to Light.
3
If that does not resolve the problem, go to Personalization - Themes, then set that to the 'Windows Default Theme.