Currently, the header component makes use of class names that do not exist as part of its component CSS file, as these are expected to be available globally. Angular CLI projects include a src/styles.css file for that purpose, to include application-wide styles.

Add the following CSS to src/styles.css:

html {
   box-sizing: border-box;
   font-size: 62.5%;  /* =10px */
 }
 
 *, *:before, *:after {
   box-sizing: inherit;
 }
 
 html, body {
   margin: 0;
   padding: 0;
   width: 100%;
   height: 100%;
 }
 
 body {
   font-size: 1.4rem;
 }
 
 .hidden {
   display: none !important;
 }
 
 .maxHeight {
   height: 100%;
 }
 
 .maxWidth {
   width: 100%;
 }
 
 .flex {
   display: flex;
 }
 
 .flex-justify-content--center {
   justify-content: center;
 }
 
 .flex-align-items--center {
   align-items: center;
 }