CSS_CUSTOM

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<!DOCTYPE html>
<html>
<head>
	<title>CSS Custom Property and Media Query</title>
	<meta name="viewport" content="width=device-width, initial-scale=1">
	<style>
		:root {
  --blue: #7ef7ed;
  --white: #ffffff; 
}

.container {
  --fontsize: 25px;
}

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

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

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

@media screen and (min-width: 450px) {
  .container {
    --fontsize: 50px;
  }
  :root {
    --blue: lightblue;
  }
}
	</style>
</head>
<body>
<div class="container">
	<p>When the browser's width is less then 450px, the font-size of this div is 15px. When it is 450px or wider, set the --font-size variable value to 35px and the value of <b>--blue</b> variable changes to lightblue. Resize the browser window to see the effect.</p>
</div>
</body>
</html>