show navigation buttons only on mouse over

I'm tring to do that with CSS but it isn't working. I've tried using:

.swiper-button-prev {  display: none;}.swiper-button-prev:hover {  display: block;}

but it doesn't effect at all. if I do add some rule like background-color: yellow it does affect the element so the CSS is applied. What am I missing? here's the full thing