After thoroughly understanding the problem you are facing and doing R&D, I suggest you switch to Custom ComposeViewController because you cannot change keyboardAppearance in ComposeViewController's .
To implement the Configure ComposeViewController follow these steps:
1) Made some kind of custom view, for example SLComposeViewController's View.
2) On the post button, you need to implement the logic in SLComposeViewControllerResultDone
[_shareComposerSheet setCompletionHandler:^(SLComposeViewControllerResult result) { switch (result) { case SLComposeViewControllerResultCancelled: break; case SLComposeViewControllerResultDone: { } break; default: break; } }];
3) . When you want to show your own view, make sure that the textView that you use in the custom view needs to set the keyboardAppearance's property to UIKeyboardAppearanceDark
self.yourTextField.keyboardAppearance = UIKeyboardAppearanceDark;
Below is an image to customize the view that was implemented in my recent project.

Hope this helps you.
Irfan
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