<div class="wrapper">
  <div class="flex">
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
  </div>
  
  <div class="grid">
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
  </div>
</div>

* {
  box-sizing: border-box;
}

:root {
  --bg: darkOrchid;
}

body {
  background-color: #212121;
  overflow-x: hidden;
}

.wrapper {
  max-width: 1200px;
  margin: 40px auto;
}

.flex, .grid {
  counter-reset: item;
}

.grid {
  --bg: orange;
}

.item {
  background-color: var(--bg);
  border-radius: 0.25em;
  counter-increment: item;
  padding: 20px;
  
  &::after {
    content: counter(item);
  }
}

/* This statement is needed for the flexbox layout */
.flex {
  display: flex;
  flex-wrap: wrap;
  margin: -10px;
  width: calc(100% + 20px);
}

/* We also need to size the flex items */
.flex .item {
  width: calc((100% / 3) - 20px);
  margin: 0 10px 20px 10px;
}

.grid {
  /* Only these 3 lines needed for the same Grid layout */
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 20px;
  
  margin-top: 50px;
}