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<!DOCTYPE html>
<html>
<head>
<style>
:root {
  --blue: #1e90ff;
  --white: #ffffff; 
}

body {
  background-color: var(--blue);
}

h2 {
  border-bottom: 2px solid var(--blue);
}

.container {
  color: var(--blue);
  background-color: var(--white);
  padding: 15px;
}

.container button  {
  background-color: var(--white);
  color: var(--blue);
  border: 1px solid var(--blue);
  padding: 5px;
}
</style>

// Create a function for getting a variable value
function myFunction_get() {
  // Get the styles (properties and values) for the root
  var rs = getComputedStyle(r);
  // Alert the value of the --blue variable
  alert("The value of --blue is: " + rs.getPropertyValue('--blue'));
}

// Create a function for setting a variable value
function myFunction_set() {
  // Set the value of variable --blue to another value (in this case "lightblue")
  r.style.setProperty('--blue', 'lightblue');
}
</script>
</head>
<body>

<h1>Get and Change CSS Variable With JavaScript</h1>

<div class="container">
  <h2>Lorem Ipsum</h2>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Etiam semper diam at erat pulvinar, at pulvinar felis blandit.</p>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Etiam semper diam at erat pulvinar, at pulvinar felis blandit.</p>
  <p>
    <button>Yes</button>
    <button>No</button>
  </p>
</div>
<br>

<button type="button" onclick="myFunction_get()">Get CSS Variable with JavaScript</button>
<button type="button" onclick="myFunction_set()">Change CSS Variable with JavaScript</button>

</body>
</html>