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How to use CSS variables

Telmo Goncalves's photo
Telmo Goncalves
·

4 min read

Since I learned about CSS variables that's all I've been using for my projects. This website aside from using TailwindCSS I'm always using CSS variables as well. Let us take a look how we can accomplish that.

Per class

One way of doing this is by a class on our <html> tag. But first, let us write some CSS to see how it goes:

/* We can look at `:root` as when <html> has no class */
:root {
  --text-color: #000;
  --background-color: #000;
}

Our HTML can look something like this:

<html>
  <head>
    <style>
      :root {
        --text-color: #000;
        --background-color: #000;
      }
    </style>
  </head>
  <body>
    Hey there!
  </body>
</html>

Let us wrap our Hey there! within a div and apply our styling colors to it:

<html>
  <head>
    <style>
      :root {
        --text-color: #000;
        --background-color: #fff;
      }

      .greetings {
        color: var(--text-color);
        background-color: var(--background-color);
      }
    </style>
  </head>
  <body>
    <div class="greetings">Hey there!</div>
  </body>
</html>

Cool, our text should be #000 (dark) and our background #fff (light).

As I've mentioned, this can be manipulated through a class on our html tag, let us change that for now:

<html class="blue">
  <head>
    <!-- ... -->
  </head>
</html>

Great, let us set our variables for when the class is blue shall we? Add the following inside our <style> tags:

[class='blue'] {
  --text-color: #fff;
  --background-color: blue;
}

So, we are switching our text color with #fff and our background with blue. If you refresh the page is should change to our new colors.

Let us create 2 more set of colors:

[class='red'] {
  --text-color: #fff;
  --background-color: red;
}

[class='green'] {
  --text-color: #fff;
  --background-color: green;
}

Awesome! Let us create a switch for this.

The Switch

To create a switch we need to do 2 things, create a few <button>s and add some JavaScript code to take care of changing the class on our html tag.

Buttons

Inside our <body> tag, right after </div> let us add a few buttons:

<button onclick="switchTheme('blue')">Blue</button>
<button onclick="switchTheme('red')">Red</button>
<button onclick="switchTheme('green')">Green</button>

So, we're creating 3 buttons with an onclick action that will call a function, let us create that function.

Function

Inside our <head> that's where we'll be adding our switchTheme() function:

<script>
  function switchTheme(className) {
    document.documentElement.className = className
  }
</script>

Simple as that! If you try it out you should see it's applying our CSS variables.

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