<section class="sec1 main">
<div id="sel_1"></div>
<div id="sel-1"></div>
<div id="sel_2"></div>
<div id="sel-2"></div>
<div id="sel_3"></div>
</section>

body {
  height: 100vh;
}
section {
  height: 100%;
}
div {
  width: 100%;
  height: 20%;
  display: block;
  background: #484848;
  margin: 2px 0;
}
.sec1.main div[id*='sel_'] {
  background: #649885;
}