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HTML 

              
                <h1>Pure CSS Color Theming 🎨</h1>

<div class="box-container">
  <div class="box-container">
    <div class="box primary"><p>Primary (Base)</p></div>
    <div class="box primary-light"><p>Primary Light<p/></div>
    <div class="box primary-dark"><p>Primary Dark</p></div>
    <div class="box primary-complement"><p>Primary Complement</p></div>
    <div class="box primary-triad-1"><p>Primary Triad 1</p></div>
    <div class="box primary-triad-2"><p>Primary Triad 2</p></div>
  </div>
  <div class="box-container">
    <div class="box secondary">Secondary (Base)</div>
    <div class="box secondary-light">Secondary Light</div>
    <div class="box secondary-dark">Secondary Dark</div>
    <div class="box secondary-complement">Secondary Complement</div>
    <div class="box secondary-triad-1"><p>Secondary Triad 1</p></div>
    <div class="box secondary-triad-2"><p>Secondary Triad 2</p></div>
  </div>
</div>



<fieldset class="controls">
  <legend>Select Base Values:</legend>
  <div>
    <input type="color" value="#6400f0" id="primary-color-input"/>
    <label for="primary-color-input">Primary</label>
  </div>
  <div>
    <input type="color" value="#018989" id="secondary-color-input"/>
    <label for="secondary-color-input">Secondary</label>
  </div>
</fieldset>
!

CSS 

              
                :root {
  /*  Base Values  */
  --primary-color-h: 265;
  --primary-color-s: 100%;
  --primary-color-l: 47%;
  --secondary-color-h: 180;
  --secondary-color-s: 99%;
  --secondary-color-l: 27%;
  --contrastThreshold: 60%;

  /*  Calculations Based on Lightness  */
  --lightnessTransform: 10%;
  --darknessTransform: 15%;
  --primary-color-light-l: calc(var(--primary-color-l) + var(--lightnessTransform));
  --primary-color-dark-l: calc(var(--primary-color-l) - var(--darknessTransform));
  --secondary-color-light-l: calc(var(--secondary-color-l) + var(--lightnessTransform));
  --secondary-color-dark-l: calc(var(--secondary-color-l) - var(--darknessTransform));
  
  --color-primary: hsl(var(--primary-color-h),
                       var(--primary-color-s),
                       var(--primary-color-l));
  --color-secondary: hsl(var(--secondary-color-h),
                       var(--secondary-color-s),
                       var(--secondary-color-l));
  --color-primary-dark:hsl(var(--primary-color-h),
                       var(--primary-color-s),
                       var(--primary-color-dark-l));
  --color-secondary-dark:hsl(var(--secondary-color-h),
                       var(--secondary-color-s),
                       var(--secondary-color-dark-l));
  --color-primary-light:hsl(var(--primary-color-h),
                       var(--primary-color-s),
                       var(--primary-color-light-l));
  --color-secondary-light:hsl(var(--secondary-color-h),
                       var(--secondary-color-s),
                       var(--secondary-color-light-l));

  /*  Calculations Based on Hue  */
  --color-primary-complement: hsl(calc(var(--primary-color-h) + 180),
                          var(--primary-color-s),
                          var(--primary-color-l));
  --color-primary-triad-1: hsl(calc(var(--primary-color-h) + 120),
                          var(--primary-color-s),
                          var(--primary-color-l));
  --color-primary-triad-2: hsl(calc(var(--primary-color-h) - 120),
                          var(--primary-color-s),
                          var(--primary-color-l));
  --color-secondary-complement: hsl(calc(var(--secondary-color-h) + 180),
                          var(--secondary-color-s),
                          var(--secondary-color-l));
  --color-secondary-triad-1: hsl(calc(var(--secondary-color-h) + 120),
                          var(--secondary-color-s),
                          var(--secondary-color-l));
  --color-secondary-triad-2: hsl(calc(var(--secondary-color-h) - 120),
                          var(--secondary-color-s),
                          var(--secondary-color-l));
}


/*  Apply Color to Boxes */

.primary {
  background: var(--color-primary);
  
  --switch: calc((var(--primary-color-l) - var(--contrastThreshold)) * -100);
  color: hsl(0, 0%, var(--switch));
}

.primary-light {
  background: var(--color-primary-light);
  --switch: calc((var(--primary-color-light-l) - var(--contrastThreshold)) * -100);
  color: hsl(0, 0%, var(--switch));
}

.primary-dark {
  background: var(--color-primary-dark);
  --switch: calc((var(--primary-color-dark-l) - var(--contrastThreshold)) * -100);
  color: hsl(0, 0%, var(--switch));
}

.primary-complement {
  background: var(--color-primary-complement);
  --switch: calc((var(--primary-color-l) - var(--contrastThreshold)) * -100);
  color: hsl(0, 0%, var(--switch));
}

.primary-triad-1 {
  background: var(--color-primary-triad-1);
  --switch: calc((var(--primary-color-l) - var(--contrastThreshold)) * -100);
  color: hsl(0, 0%, var(--switch));
}

.primary-triad-2 {
  background: var(--color-primary-triad-2);
  --switch: calc((var(--primary-color-l) - var(--contrastThreshold)) * -100);
  color: hsl(0, 0%, var(--switch));
}

.secondary {
  background: var(--color-secondary);
  --switch: calc((var(--secondary-color-l) - var(--contrastThreshold)) * -100);
  color: hsl(0, 0%, var(--switch));
}

.secondary-light {
  background: var(--color-secondary-light);
  --switch: calc((var(--secondary-color-light-l) - var(--contrastThreshold)) * -100);
  color: hsl(0, 0%, var(--switch));
}

.secondary-dark {
  background: var(--color-secondary-dark);
  --switch: calc((var(--secondary-color-dark-l) - var(--contrastThreshold)) * -100);
  color: hsl(0, 0%, var(--switch));
}

.secondary-complement {
  background: var(--color-secondary-complement);
  --switch: calc((var(--secondary-color-l) - var(--contrastThreshold)) * -100);
  color: hsl(0, 0%, var(--switch));
}

.secondary-triad-1 {
  background: var(--color-secondary-triad-1);
  --switch: calc((var(--secondary-color-l) - var(--contrastThreshold)) * -100);
  color: hsl(0, 0%, var(--switch));
}

.secondary-triad-2 {
  background: var(--color-secondary-triad-2);
  --switch: calc((var(--secondary-color-l) - var(--contrastThreshold)) * -100);
  color: hsl(0, 0%, var(--switch));
}

/*  Etc */
.box-container {
  display: flex;
  flex-wrap: wrap;
  width: 100%;
  max-width: 1100px;
  margin: 0 auto;
  justify-content: center;
  
}

.controls {
  margin-top: 1rem;
  padding: 1rem 2rem;
  border-color: #bbb;
  border-style: dashed;
}

.controls div {
  width: 100px;
  text-align: center;
  display: inline-block;
}

legend {
  padding: 0 1rem;
}

.box {
  height: 100px;
  flex: 1;
  flex-basis: 100px;
  display: flex;
  justify-content: center;
  align-items: center;
  margin: 0.5rem;
  position: relative;
  border-radius: 10px;
  text-align: center;
  padding: 0 0.25rem;
}

body {
  font-family: 'Roboto', serif;
  padding: 0;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  font-family: 'Roboto Mono', monospace;
}

html {
  min-height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
}

input[type="color"] {
  border: 2px solid;
  padding: 0;
  vertical-align: middle;
  width: 42px;
  height: 42px;
  border-radius: 50%;
  overflow: hidden;
  cursor: pointer;
}

input[type="color"]::-webkit-color-swatch-wrapper {
	padding: 0;
}
input[type="color"]::-webkit-color-swatch {
	border: none;
}
!

JS 

              
                // This is only for the Inputs to work dynamically

function setTheme(H, inputType) {
  // Convert hex to RGB first
  let r = 0, g = 0, b = 0;
  if (H.length == 4) {
    r = "0x" + H[1] + H[1];
    g = "0x" + H[2] + H[2];
    b = "0x" + H[3] + H[3];
  } else if (H.length == 7) {
    r = "0x" + H[1] + H[2];
    g = "0x" + H[3] + H[4];
    b = "0x" + H[5] + H[6];
  }
  // Then to HSL
  r /= 255;
  g /= 255;
  b /= 255;
  let cmin = Math.min(r,g,b),
      cmax = Math.max(r,g,b),
      delta = cmax - cmin,
      h = 0,
      s = 0,
      l = 0;

  if (delta == 0)
    h = 0;
  else if (cmax == r)
    h = ((g - b) / delta) % 6;
  else if (cmax == g)
    h = (b - r) / delta + 2;
  else
    h = (r - g) / delta + 4;

  h = Math.round(h * 60);

  if (h < 0)
    h += 360;

  l = (cmax + cmin) / 2;
  s = delta == 0 ? 0 : delta / (1 - Math.abs(2 * l - 1));
  s = +(s * 100).toFixed(1);
  l = +(l * 100).toFixed(1);
  
  document.documentElement.style.setProperty(`--${inputType}-color-h`, h);
  document.documentElement.style.setProperty(`--${inputType}-color-s`, s + '%');
  document.documentElement.style.setProperty(`--${inputType}-color-l`, l + '%');
}

const inputs = ['primary', 'secondary'];

inputs.forEach((inputType) => {
  document.querySelector(`#${inputType}-color-input`)
    .addEventListener('change', (e) => {
    setTheme(e.target.value, inputType)
  });
});
!
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