Hi Andrew,
This should do it:
.rev_slider_wrapper .tp-rightarrow, .rev_slider_wrapper .tp-rightarrow.large, .rev_slider_wrapper .tp-rightarrow.square, .rev_slider_wrapper .tp-rightarrow.round, .rev_slider_wrapper .tp-rightarrow.default,
.rev_slider_wrapper .tp-leftarrow, .rev_slider_wrapper .tp-leftarrow.large, .rev_slider_wrapper .tp-leftarrow.square, .rev_slider_wrapper .tp-leftarrow.round, .rev_slider_wrapper .tp-leftarrow.default{
opacity: 0.5;
}
.rev_slider_wrapper .tp-rightarrow:hover, .rev_slider_wrapper .tp-rightarrow.large:hover, .rev_slider_wrapper .tp-rightarrow.square:hover, .rev_slider_wrapper .tp-rightarrow.round:hover, .rev_slider_wrapper .tp-rightarrow.default:hover,
.rev_slider_wrapper .tp-leftarrow:hover, .rev_slider_wrapper .tp-leftarrow.large:hover, .rev_slider_wrapper .tp-leftarrow.square:hover, .rev_slider_wrapper .tp-leftarrow.round:hover, .rev_slider_wrapper .tp-leftarrow.default:hover {
opacity: 1;
}