How to resolve Facebook dark mode problems on iPadOS 15.8?

It is a Facebook problem and that is why you are only having that trouble when using Facebook. They use their own Dark Mode option and have done a poor job implementing it to work correctly for all users. That is their job and they are the only ones that can fix it. Let them know. The issue first started 4 years ago and Facebook resolved the problem at that time, but it has resurfaced frequently when they update the app and also change the website configuration.

If you look at the update history of Facebook in the App Store, you will see it has been updated 10 times in just the last month and the most recent one was 5 days ago. I suspect you noticed this problem with one of those Facebook updates. If your iPad Air 2 is fully updated, the last update to iPadOS 15.8.4 was 4 months ago and that is also the only time Safari is updated, so I think you can rule those out as the cause, since you said the issue suddenly appeared.

The Facebook website can be modified at any time and they do not publish any change logs to let you know when that has happened.

No, it is a Facebook problem, unless all of your apps and the Home screen went into dark mode. If it’s just one app, the problem is with the app.