Replace default navigation arrows with Dashicons icons.

envira-lightbox-dashicons-nav-arrows.css

/*** Make nav arrows always visible (optional) ***/
.envirabox-nav { visibility: visible !important; }

/* Remove default navigation arrow background images */
.envirabox-nav span { display: none !important; }

/* Make the clickable nav area larger */
.envirabox-nav { width: 30% !important; height: 100% !important; }

/* Add custom Dashicons nav arrows */
.envirabox-nav::before { 
  display: block;
  font-family: 'Dashicons';
  position: absolute;
  top: 50%;
  
  /*** Color of the icons, modify as desired ***/
  color: #fff !important;
  
  /*** Font size of the icons, modify as desired ***/
  font-size: 50px;
}

/* Prev nav arrow */
.envirabox-prev::before {
  /*** Replace with the Dashicons icon unicode you want to use ***/
  content: '\f341';
  
  left: 0 !important;
  -webkit-transform: translate(50%, -50%);
  -moz-transform: translate(50%, -50%);
  transform: translate(50%, -50%);
}

.envirabox-next::before {
  /*** Replace with the Dashicons icon unicode you want to use ***/
  content: '\f345';
  
  right: 0 !important;
  -webkit-transform: translate(-50%, -50%);
  -moz-transform: translate(-50%, -50%);
  transform: translate(-50%, -50%);
}