Hi mutiaradiandini,
It sounds like only your WhatsApp keyboard is in light mode, while the rest of the phone is in dark mode. We're glad you've joined us in Apple Support Communities. Let's see if this helps.
If you haven't already, let's follow the steps in If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open. The troubleshooting in it will commonly fix issues of apps not working correctly on your iPhone.
If the issue remains after following all of those steps, you'll want to reach out to the app developer for help. Since this isn't happening with any other app, it's isolated to WhatsApp. Their development team will have the best information about their specific app. Here's How to contact an app developer.
We hope that helps with your question and take care!