Hello Suellen10,
Is the issue affecting all apps? Checking various apps to see if they've switched to Light Mode can help to see if the issue is specific to a particular app.
Also try checking in the accessibility settings at Settings > Accessibility > Display & Text Size, and see if any are enabled, such as Classic Invert, Smart Invert, or others such as Color Filters. If so, try disabling them and see if that's helped.
In Settings > Display & Brightness try checking to see if Night Shift or True Tone are on, or if Night Shift is scheduled to be on. If so, try turning them off there and check again to see if things are resolved.
Use Dark Mode on your iPhone and iPad - Apple Support
Cheers.