<div class="swiper-container">
    <div class="swiper-wrapper">
      <div class="swiper-slide">Slide 1</div>
      <div class="swiper-slide">Slide 2</div>
      <div class="swiper-slide">Slide 3</div>
      <div class="swiper-slide">Slide 4</div>
      <div class="swiper-slide">Slide 5</div>
      <div class="swiper-slide">Slide 6</div>
      <div class="swiper-slide">Slide 7</div>
      <div class="swiper-slide">Slide 8</div>
      <div class="swiper-slide">Slide 9</div>
      <div class="swiper-slide">Slide 10</div>
      <div class="swiper-slide">Slide 11</div>
      <div class="swiper-slide">Slide 12</div>
      <div class="swiper-slide">Slide 13</div>
      <div class="swiper-slide">Slide 14</div>
      <div class="swiper-slide">Slide 15</div>
      <div class="swiper-slide">Slide 16</div>
      <div class="swiper-slide">Slide 17</div>
      <div class="swiper-slide">Slide 18</div>
      <div class="swiper-slide">Slide 19</div>
      <div class="swiper-slide">Slide 20</div>
    </div>
    <!-- Add Pagination -->
    <div class="swiper-pagination"></div>
  </div>

    html, body {
      position: relative;
      height: 100%;
    overflow: hidden;
    }
    body {
      background: #eee;
      font-family: Helvetica Neue, Helvetica, Arial, sans-serif;
      font-size: 14px;
      color:#000;
      margin: 0;
      padding: 0;

    }
    .swiper-container {
      width: 100%;
      height: 100%;
      margin-left: auto;
      margin-right: auto;
    }
    .swiper-slide {
      text-align: center;
      font-size: 18px;
      background: #fff;
      height: 100%;
    opacity: 0.3;

      /* Center slide text vertically */
      display: -webkit-box;
      display: -ms-flexbox;
      display: -webkit-flex;
      display: flex;
      -webkit-box-pack: center;
      -ms-flex-pack: center;
      -webkit-justify-content: center;
      justify-content: center;
      -webkit-box-align: center;
      -ms-flex-align: center;
      -webkit-align-items: center;
      align-items: center;
     transition: .3s all;
    }

.swiper-slide_active {
  opacity: 1;
  background-color: orange;
}

var swiper = new Swiper(".swiper-container", {
  slidesPerView: 6,
  spaceBetween: 30,
  loop: true,
  pagination: {
    el: ".swiper-pagination",
    clickable: true
  },
  on: {
  init:  function () {
       swiperClassHandler (this.activeIndex, '.swiper-slide')
    },
    slideChange: function () {
       swiperClassHandler (this.activeIndex, '.swiper-slide')
    },
  },
});

function swiperClassHandler (i, csl) {
     let sliders = document.querySelectorAll(csl);
    sliders.forEach(n => n.classList.remove('swiper-slide_active'));
     console.log(sliders);
     Array.prototype.slice.call(sliders, i + 2 , i  + 4).forEach(n => n.classList.add('swiper-slide_active'));
}