I've added the css for changing the color of the arrows and the tgransparent background.

It only appears to be changing the first one

Using the following css

.swiper-container .slider-prev > i {  color: black; }
.swiper-container .slider-next > i { color: black; }

.swiper-container a.slider-prev:hover, .swiper-container a.slider-next:hover {
    background-color: rgba(0,0,0,0.8);
}
.swiper-container a.slider-prev, .swiper-container a.slider-next {
    background-color: rgba(0,0,0,0.2);

Is the 'i' before the colour braces the issue?

Thanks Tahir