Check out example codes for "css stylesheet template". It will help you in understanding the concepts better.

Code Example 1

<!DOCTYPE html>
<html>
<body>

<h1>My First Heading</h1>
<p>My first paragraph.</p>

</body>
</html>

Code Example 2

<!doctype html>
<!--[if IE 8]><html lang="pt-br" class="no-js ie8 oldie"><![endif]-->
<!--[if IE 9]><html lang="pt-br" class="no-js ie9"><![endif]-->
<!--[if gt IE 9]><!-->
<html lang="pt-br" class="no-js">
<!--<![endif]-->
<head>
	<base href="">
	<meta charset="utf-8"/>
	<meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0"/>
	<meta name="apple-mobile-web-app-status-bar-style" content="black-translucent"/>
	<meta name="apple-mobile-web-app-capable" content="yes"/>
	<meta name="theme-color" content="#111111" />
	<meta name="description" content=""/>
	<title></title>
	<link rel="author" href="humans.txt">
	<link rel="manifest" href="manifest.json" />
	<link rel="apple-touch-icon" sizes="114x114" href="apple-touch-icon-114x114-precomposed.png">
	<link rel="apple-touch-icon" sizes="72x72" href="apple-touch-icon-72x72-precomposed.png">
	<link rel="apple-touch-icon" sizes="57x57" href="apple-touch-icon-57x57-precomposed.png">
	<link rel="apple-touch-icon" sizes="57x57" href="apple-touch-icon.png">
	<link rel="shortcut icon" sizes="16x16" href="favicon.ico">
	<link rel="stylesheet" media="all" href="styles/index.css">
</head>
<body>
	<!-- fallback -->
	<noscript>You need to enable JavaScript to run this app.</noscript>
	
	<!-- app -->
	<main id="root" role="main"></main>
	
	<!-- libraries - scripts at the bottom for fast page loading -->
	<script src="scripts/index.js"></script>
</body>
</html>

Code Example 3

/* Answer to: "free css templates" */

/*
  You can find over 78 hundred free HTML templates here:
  https://colorlib.com/wp/free-css-website-templates/

  That's not enough? Well, you have find 548 more here:
  https://templatemo.com/

  You don't like any of them?? Well, there's over 3030 here:
  https://www.free-css.com/free-css-templates

  You don't like them either??? Okay! I've had enough!
  You can learn how to make your own website, over here:
  https://www.w3schools.com/css/
*/

Code Example 4

<!-- Answer to: "html-answer" -->

<!--
  You can find over 100 hundred free HTML templates here:
  https://colorlib.com/wp/free-html-website-templates/

  That's not enough? Well, you have find 800 more here:
  https://colorlib.com/wp/templates/

  You don't like any of them?? Well, there's over 2000 here:
  https://mobirise.com/html-templates/

  You don't like them either??? Okay! I've had enough!
  You can learn how to make your own website, over here:
  https://www.w3schools.com/html/
-->

Code Example 5

/* Answer to "css stylesheet template" */

/*
  You can find many here:
  https://www.w3schools.com/css/css_templates.asp

  Here's one I edited myself!
*/

<!DOCTYPE html>
<html lang="en">
<head>
<title>CSS Template</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">

/* CSS: */
<style>
* {
  box-sizing: border-box;
}

body {
  font-family: Arial, Helvetica, sans-serif;
}

/* Style the header */
.header {
  background-color: #ddd;
  padding: 30px;
  text-align: center;
  font-size: 35px;
}

/* Container for flexboxes */
.row {
  display: -webkit-flex;
  display: flex;
}

/* Create three unequal columns that sits next to each other */
.column {
  padding: 10px;
  height: 300px; /* Should be removed. Only for demonstration */
}

/* Left and right column */
.column.side {
   -webkit-flex: 1;
   -ms-flex: 1;
   flex: 1;
}

/* Middle column */
.column.middle {
  -webkit-flex: 3;
  -ms-flex: 3;
  flex: 3;
}

/* Style the footer */
.footer {
  background-color: #f1f1f1;
  padding: 10px;
  text-align: center;
}

/* Responsive layout - makes the three columns stack on top of each other instead of next to each other */
@media (max-width: 600px) {
  .row {
    -webkit-flex-direction: column;
    flex-direction: column;
  }
}
</style>

</head>
<body>

<h2>CSS Template using Flexbox</h2>
<p>In this example, we have created a header, three unequal columns and a footer. On smaller screens, the columns will stack on top of each other. Resize the browser window to see the responsive effect.</p>
<p><strong>Note:</strong> Flexbox is not supported in Internet Explorer 10 and earlier versions.</p>

<div class="header">
  <h2>Header</h2>
</div>

<div class="row">
  <div class="column side" style="background-color:#aaa;">Column</div>
  <div class="column middle" style="background-color:#bbb;">Column</div>
</div>

<div class="footer" style="background-color:#ccc;">
  <p>Footer</p>
</div>

</body>
</html>

Code Example 6

<!DOCTYPE>
<html>
  <head>
    <title>Title</title>
    
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" href="style.css">
  </head>
  
  <body>
    
  </body>
</html>

Learn ReactJs, React Native from akashmittal.com