Lack of contrast on hover of lightbox buttons when expanded

When opening up the lightbox modal, a dark section covers the whole screen.
Transparent buttons are displayed, for different actions.
In Iceberg, those buttons turn dark grey/black when hovered and become invisible.
Here is a demonstration of the issue:
https://up1.xwikisas.com/#tf5RT3ROu2qxMoG2hIXfiA
Note that the green border around the buttons is added by a firefox extension to highlight hovered/focus. Without it, the user would see barely anything when hovering.