JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box">
    <h1><span>Moderní webdesign</span> za nejnižší ceny!</h1>
</div>

CSS

.box {
  float: left;
  margin: 250px 0 0 0;
  height: 35%;
  width: 50%;
  background-image: url('img/header-bcg.png');
}
.box h1 {
  font-size: 40px;
  font-family: Bree serif;
  color: 33adae;
  margin-top: 30px;
  margin-right: 15px;
  line-height: 10px;
  float: right;
}
@media screen and (max-width: 768px) {
  .section1 {
    width: 100%;
  }
  .box {
    width: 100%;
  }
  .box h1 {
    text-align: center;
    margin: 0 auto;
      line-height: 1em;
  }
    
  footer,
  nav {
    width: 100%;
  }
}