<div class="container">
  <div class="block-element">
    Block element (div) to be horizontally centered
  </div>
</div>

@import url('https://fonts.googleapis.com/css2?family=Open+Sans&display=swap');

:root {
  --const-color-black: #000000;
  --const-color-white: #FFFFFF;
  --color-header: #D9A443;
  --color-main-background: #F9F6F1;
  --color-link: #375F9B;
}

body {
  font-family: 'Open Sans', sans-serif;
  color: var(--const-color-black);
}

* {
  box-sizing: border-box;
}

.container {
  background-color: var(--color-main-background);
  border: 1px solid var(--color-link);
  margin: 1.0rem auto;
  padding: 1.0rem 0;
  width: 100%;
}

.block-element {
  background-color: var(--color-link);
  color: var(--const-color-white);
  max-width: 100%;
  padding: 1.0rem;
  text-align: center;
  
  /* Set block element to fixed width */
  width: 400px;
  
  /* Horizontally center block element */
  margin: 0 auto;
}