Made an update that sets the caret color to match the text color.
https://github.com/discourse/discourse/commit/54db23e4fff759e716f56d4f9582e22652a66763
I was considering having an exception for light themes on iOS, but that seemed like a very narrow exception and there are cases where you could have a blue background with a blue iOS caret… so it seems like it would be better to consistently match the text color everywhere…
I wonder if Apple will have a better cursor color for dark mode in iOS 13? 
You can override this in a theme with the following CSS:
input,
select,
textarea {
caret-color: /* your color here */;
}