Add this into the Custom CSS box in your Salient Theme Options panel (We write all Custom CSS in the Live Browser to ensure accuracy. If you cannot see any changes, make sure there is no red cross in the Custom CSS Box as any syntax error would cause all CSS under that line of code to not show up on the Frontend):

.bottom_controls #portfolio-nav ul:first-child #all-items a i {
    background-color: #fff !important;
    box-shadow: -.5em 0 #fff, -.5em .5em #fff, .5em 0 #fff, .5em -.5em #fff, 0 -.5em #fff, -.5em -.5em #fff, 0 .5em #fff, .5em .5em #fff;
}

#portfolio-nav a i, #portfolio-nav a i {
    color: #fff !important;
}

#portfolio-nav #prev-link a, #portfolio-nav #next-link a {
    color: #fff !important;
}

Thanks