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HTML

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About the <head>

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CSS

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

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JavaScript

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JavaScript preprocessors can help make authoring JavaScript easier and more convenient. For instance, CoffeeScript can help prevent easy-to-make mistakes and offer a cleaner syntax and Babel can bring ECMAScript 6 features to browsers that only support ECMAScript 5.

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HTML 

              
                <!--
This div defines the theme color variable in an inline style.

This inline style is then changed in the JS to change the theme color.
-->
<div class="container" style="--theme-color: #e00">
  <nav>
    <h1>Site Title</h1>
    <ul class="links">
      <li>Link 1</li>
      <li>Link 2</li>
      <li>Link 3</li>
    </ul>
  </nav>
  <div class="change_color">
    <h1>Change Theme Color:</h1>
    <p>Clicking each of the buttons changes the "--theme-color" CSS variable through JS.</p>
    <button class="active" id="redBtn"></button>
    <button id="greenBtn"></button>
    <button id="blueBtn"></button>
  </div>
  <div class="content">
    <h1>Some Content</h1>
    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur</p>
  </div>
</div>
!

CSS 

              
                /*
Demo from an article on my personal website about the CSS Variables: https://xtrp.io/blog/2020/01/27/css-variables-explained-with-demo/.

Give this CodePen a ❤️ if you liked it and check out the article :)
*/

/* global styles */
* {
  margin: 0;
  padding: 0;
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
}

/* style container */
div.container {
  width: 100%;
  padding: 0 calc((100% - 750px) / 2);
  box-sizing: border-box;
}

@media only screen and (max-width: 775px) {
  div.container {
    padding: 1.5rem;
  }
}

/* style nav with CSS variable as color */
nav {
  height: 3.5rem;
  width: 100%;
  display: block;
  border-bottom: 1px solid #cccccc;
  
  /* set color to theme color */
  color: var(--theme-color);
}

nav h1 {
  font-size: 1.55rem;
  line-height: calc(3.5rem + 1px);
  letter-spacing: -.03rem;
  float: left;
}

nav ul.links {
  float: right;
  height: 3.5rem;
  overflow: hidden;
}
nav ul.links li {
  display: inline-block;
  font-weight: 600;
  line-height: calc(3.5rem + 1px);
  cursor: pointer;
}
nav ul.links li:not(:last-child) {
  margin-right: 1rem;
}

/* content divs */
div.container > div {
  margin: 2rem 0;
  padding: 1.5rem;
  
  /* background is theme color CSS variable */
  background-color: var(--theme-color);
  
  color: #fff;
  border-radius: 4px;
  box-shadow: 0 .25rem 1rem rgba(0, 0, 0, .2);
}
div.container > div h1 {
  font-size: 2rem;
  line-height: 1.35;
  margin-bottom: .75rem;
}
div.container > div p {
  line-height: 1.5rem;
  font-size: .9rem;
}

/* change color div */
div.container > div.change_color {
  background-color: #ffffff;
  
  /* border color is theme color CSS variable */
  border: 3px solid var(--theme-color);
  
  /* color is theme color CSS variable */
  color: var(--theme-color);
  
  text-align: center;
}
div.container > div.change_color h1,
div.container > div.change_color p {
  text-align: left;
  margin-bottom: .5rem;
}
div.container > div.change_color p {
  margin-bottom: 1rem;
}
div.change_color button {
  width: 3.25rem;
  height: 3.25rem;
  border: none;
  margin: .5rem;
  box-shadow: 0 .1rem .5rem rgba(0, 0, 0, .1);
  border-radius: 4px;
  cursor: pointer;
  box-sizing: border-box;
  border: 3px solid transparent;
  outline: none;
}
div.change_color button.active {
  border-color: #000;
}
div.change_color button#redBtn {
  background-color: #e00;
}
div.change_color button#blueBtn {
  background-color: #00e;
}
div.change_color button#greenBtn {
  background-color: #0e0;
}
!

JS 

              
                // remove active class from active btn
const removeActive = () => {
  document.querySelector("button.active").classList.remove("active");
}

// button click events
const redBtn = document.querySelector("button#redBtn");
const greenBtn = document.querySelector("button#greenBtn");
const blueBtn = document.querySelector("button#blueBtn");
const container = document.querySelector("div.container");


redBtn.addEventListener("click", () => {
  removeActive();
  redBtn.classList.add("active");
  
  // change CSS variable
  document.querySelector("div.container").setAttribute("style", "--theme-color: #e00");
});

greenBtn.addEventListener("click", () => {
  removeActive();
  greenBtn.classList.add("active");
  
  // change CSS variable
  document.querySelector("div.container").setAttribute("style", "--theme-color: #0e0");
});

blueBtn.addEventListener("click", () => {
  removeActive();
  blueBtn.classList.add("active");
  
  // change CSS variable
  document.querySelector("div.container").setAttribute("style", "--theme-color: #00e");
});
!
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