.parent {
  display: flex; /* Enables Flexbox */
  height: 300px; /* Set a specific height for the parent div */
  border: 1px solid #000;
}

.child {
  background-color: lightblue;
  flex: 1; /* Optional: Makes the child take up remaining space */
}

​/*
Theme Name:     Twenty Seventeen Child
Theme URI:      https://wordpress.org/themes/twentyseventeen-child/
Description:    Tema Child per il tema Twenty Thirteen
Author:         Bruna Carolina Santos
Author URI:     
Template:       twentyseventeen
Version:        2.8-wpcom
*/