JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
  <li>1</li>
  <li>2</li>
  <li>3</li>
  <li>4</li>
  <li>5</li>
  <li>6</li>
  <li>7</li>
  <li>8</li>
  <li>9</li>
  <li>10</li>
  <li>11</li>
  <li>12</li>
  <li>13</li>
  <li>14</li>
  <li>15</li>
</ul>

CSS

ul {
  display: flex;
  flex-direction: row;
  flex-wrap: wrap;
}
li {
  width: 20%;
  height: 100px;
  display: flex;
  background: #000;
  color: #FFF;
  justify-content: center;
  align-items: center;
  align-content: center;
  font-size: 30px;
  font-family: sans-serif;
}
li.adding {
  background: #FFF;
}

JavaScript

$('ul li').after('<li class=adding></li>');