I misunderstood Oriol, and what he really wanted was to have the arrows
on top of the image, not just move the arrows from below to above. Thus,
I no longer need the carrousel to be a flex because the buttons are now
absolutely position (slick.css already sets the container to relative
position).
These arrows need to be visually inside a single container, to have a
white background, but Slick adds the two arrows separately. I had to
move them close together, remove the radius from the “common edge”, and
use padding to “move” the arrows, rather than translation, in order to
avoid showing a gap.