call id of div html in javascript code example

Example 1: html id

<style>
#grepperHeader {
  background-color: lightblue;
  color: navy;
  padding: 40px;
  text-align: center;
}
</style>

<h1 id="grepperHeader">grepperHeader</h1>

Example 2: referance html id css

<style>
  #test {
  /*Styleing here*/
  }
</style>
<h1 id="test"></h1>