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CSS

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HTML 

              
                <div class="tools">
  <label for="font-picker-size">Font size</label>
  <input type="range" id="font-picker-size" value="125" min="80" max="300">
  <label for="color-picker-primary">Primary colour</label>
  <input type="color" id="color-picker-primary" value="#b30027">
  <label for="color-picker-contrast">Contrast colour</label>
  <input type="color" id="color-picker-contrast" value="#ffffff">
</div>


<div class="template">
  <div class="primary--fill masthead">
    <h1 class="contrast--text">Using CSS Custom Properties and the CSSOM to create real&#8209time theme previews</h1>
  </div>

  <div class="body">
    <h2 class="primary--text">My themed page title</h2>
    <p>Bacon ipsum dolor amet swine t-bone leberkas, pork belly strip steak tongue shankle bresaola ball tip ribeye ham pork loin ham hock. Pork belly capicola ground round t-bone chicken doner beef ribs tail pork chop tenderloin swine flank ham. Jerky sirloin tongue <a class="primary--text" href="#">spare ribs t-bone</a> shoulder short ribs chuck frankfurter. Shoulder pig capicola landjaeger beef ribs, pork loin short ribs jerky salami cupim. Beef ribs bacon tongue, ribeye short loin turducken swine sirloin porchetta capicola picanha ball tip cupim shank frankfurter. Shank bacon biltong, flank beef turkey porchetta sausage meatloaf pastrami shankle tenderloin spare ribs short ribs pork belly.</p>

    <a class="button primary--fill contrast--text" href="#">Button</a>

    <p>Bresaola bacon shank salami pancetta drumstick andouille shoulder capicola hamburger corned beef prosciutto t-bone chuck. <a class="primary--text" href="#">Biltong sausage boudin, brisket turducken</a> doner venison landjaeger salami picanha tri-tip pork belly spare ribs short ribs. Turkey pork belly shoulder boudin tail, beef pork loin ground round spare ribs ribeye tri-tip landjaeger bresaola. Cow shankle rump tail. Ground round ham hock capicola landjaeger sausage. Ribeye pig meatball, spare ribs pancetta capicola doner salami.</p>

  </div>
</div>
!

CSS 

              
                /* themed properties
---------------------------------------------------- */

:root {
  --theme-contrast: #ffffff;
  --theme-primary: #b30027;
  --theme-font-size: 125%;
  --unsupported-message-display: none;
}


/* themed elements
---------------------------------------------------- */

.template {
  font-size: var(--theme-font-size);
}


.primary--text {
  color: var(--theme-primary);
}
.primary--fill {
  background-color: var(--theme-primary);
}


.contrast--text {
  color: var(--theme-contrast);
}
.contrast--fill {
  background-color: var(--theme-contrast);
}


/* demo styles 
---------------------------------------------------- */

body {
  font: 100% / 1.5 Arial;
  margin: 0;
  padding: 0;
}

p {
  margin: 3em 0;
}

.masthead {
  padding: 12% 2em;
  text-align: center;
  background-image: linear-gradient(rgba(0,0,0,0), rgba(0,0,0,0.3))
}

.body {
  margin: auto;
  max-width: 760px;
  padding: 2em;
}

/* Button styles
---------------------------------------------------- */
.button {
  margin: 0;
  padding: .5em 2em;
  border-radius: .25em;
  text-decoration: none;
  box-shadow: 0 .1em .2em 0em rgba(0,0,0,0.5);
  border-bottom: .15em solid rgba(0,0,0,0.25);
}

.button:hover {
  background-image: linear-gradient(
    rgba(255,255,255,0.3),
    rgba(255,255,255,0.3)
  );
}


/* Toolbox styles
---------------------------------------------------- */
.tools {
  position: fixed;
  background: rgba(225,225,225,.75);
  padding: 1em;
  margin: .5em;
  border-radius: .25em;
  max-width: 20em;
}

.tools > * {
  display: inline-block;
}

.tools label {
  min-width: 9em;
  max-width: 11em;
}


/* Show a message in unsupported browsers
---------------------------------------------------- */
:root::before {
  font: 100%/1.5 Menlo, monospace;
  content: 'NOTE: This browser doesn\'t support CSS Custom Properties';
  text-align: center;
  background: #fc0;
  padding: 1em;
  border-radius: 1em;
  position: fixed;
  left: 50%;
  bottom: 0.5em;
  transform: translate(-50%,-50%);
}

@supports (font-size: var(--theme-font-size)) {
  :root::before {
    display: none;
  }
}
!

JS 

              
                var primaryColorPicker = document.getElementById('color-picker-primary');
var contrastColorPicker = document.getElementById('color-picker-contrast');
var fontSizePicker = document.getElementById('font-picker-size');

function setThemeVar(name, value, unit) {
  var rootStyles = document.styleSheets[0].cssRules[0].style;
  rootStyles.setProperty('--theme-' + name, value + (unit || ''));
}

primaryColorPicker.addEventListener('input', function (e) {
  setThemeVar('primary', e.target.value);
});

contrastColorPicker.addEventListener('input', function (e) {
  setThemeVar('contrast', e.target.value);
});

fontSizePicker.addEventListener('input', function (e) {
  setThemeVar('font-size', e.target.value, '%');
});

setThemeVar('primary', primaryColorPicker.value);
setThemeVar('contrast', contrastColorPicker.value);
setThemeVar('font-size', fontSizePicker.value, '%');
!
999px

Console