Lightbox "X" has black background on black player

It is my first time using a lightbox slide for "Navigation Help" that should appear on every slide throughout a training. However, on the modern dark player, the X has a dark background will make it virtually impossible for learners to see. I've read other conversations that suggest this is supposed to be a red "X" but that's not what I'm seeing. Is there any fix for this aside from changing my player to the light feature (in which case I'd have to update all my screenshots).