<div class="background">
  <div class="container" data-container>
    <div class="card" data-card style="background-image: url('https://source.unsplash.com/random?1')">1</div>
    <div class="card" style="background-image: url('https://source.unsplash.com/random?2')">2</div>
    <div class="card" style="background-image: url('https://source.unsplash.com/random?3')">3</div>
    <div class="card" style="background-image: url('https://source.unsplash.com/random?4')">4</div>
    <div class="card" style="background-image: url('https://source.unsplash.com/random?5')">5</div>
    <div class="card" style="background-image: url('https://source.unsplash.com/random?6')">6</div>
  </div>
  <div class="buttons">
    <button data-remove>–</button>
    <button data-add>+</button>
  </div>
</div>

.container {
  background: #FF678C;
  box-sizing: border-box;
  display: flex;
  flex-flow: column wrap;
  height: 80vh;
  margin-bottom: 5vh;
  text-align: center;
  text-transform: uppercase;
  width: 90vw;
}

.card {
  align-items: center;
  background: white;
  background-size: cover;
  background-position: center;
  border: 5px solid white;
  box-sizing: border-box;
  color: white;
  display: flex;
  flex-grow: 0;
  font-family: sans-serif;
  font-size: 30px;
  font-weight: bold;
  height: 50%;
  justify-content: center;
  outline: 5px solid white;
  text-shadow: 0 0 5px black;
  
  // Setting the heights of the elements so they meet
  // nicely at the bottom of the container
  &:nth-child(6n - 5) {
    height: 25%;
  }
  
  &:nth-child(6n-4) {
    height: 75%;
  }
  
  &:nth-child(6n-3) {
    height: 50%;
  }
  
  &:nth-child(6n-2) {
    height: 50%;
  }
  
  &:nth-child(6n-1) {
    height: 66.666%;
  }
  
  &:nth-child(6n) {
    height: 33.333%;
  }
}

// Presentational Styles

body {
  margin: 0;
}

.background {
  align-items: center;
  background: #FF678C;
  display: flex;
  flex-direction: column;
  height: 100vh;
  justify-content: center;
  width: 100vw;
}

button {
  background: white;
  border: 2px solid transparent;
  border-radius: 0;
  color: #FF648A;
  cursor: pointer;
  font-size: 24px;
  font-weight: bold;
  padding: 10px 20px;
  transition: background-color 0.2s ease, border-color 0.2s ease, color 0.2s ease;
  
  &:hover {
    background-color: #FF648A;
    border: 2px solid white;
    color: white;
  }
}

// This JS is not needed for the layout to work.
// All it does it add/remove images.

const add = document.querySelector('[data-add]');
const remove = document.querySelector('[data-remove]');
const card = document.querySelector('[data-card]');
const container = document.querySelector('[data-container]');
let number = 7;

add.addEventListener('click', () => {
  const addCard = card.cloneNode(true);
  const image = `url('https://source.unsplash.com/random?${Math.floor(Math.random() * 100)}&w=600&q=80')`;
  addCard.style.backgroundImage = image;
  addCard.innerHTML = number;
  container.appendChild(addCard);
  number++;
});

remove.addEventListener('click', () => {
  if (container.childElementCount > 0) {
    container.removeChild(container.lastElementChild);
  }
  
  if (number > 1) {
    number--;
  }
});