When you flag an answer that has a very low score (and therefore appears greyed-out), the flag dialog is shown in the same faded-out colors:
Obviously, that doesn't make sense.
This is what it actually should look like:
For those implementing, it is probably enough to set the color of the whole dialog div to black (which is what I did in my preview). But you probably know where to look at and there are a couple of other CSS properties to fix here.