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CSS

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HTML 

              
                <main data-theme="triad" data-colors>
  <h1>Cascading Color Palette</h1>

  <menu>
    <label for="theme-select">
      <span class="label">theme:</span>
      <select name="theme-select" id="theme-select">
        <option value="default-pink">default-pink</option>
        <option value="default-blue">default-blue</option>
        <option value="black">black &amp; white</option>
        <option value="complement">complement</option>
        <option value="triad">triad</option>
        <option value="adjacent" selected>adjacent</option>
        <option value="monochrome">monochrome</option>
      </select>
    </label>
    <button data-toggle>invert</button>
    
    <label for="hue-select">
      <span class="label">hue:</span>
      <input id="hue-select" type="range" min="0" max="360" value="330">    
    </label>
    <label for="sat-select">
      <span class="label">saturation:</span>
      <input id="sat-select" type="range" min="0" max="100" value="80">
    </label>
    <label for="light-select">
      <span class="label">lightness:</span>
      <input id="light-select" type="range" min="25" max="75" value="50">
    </label>
    <label for="contrast-select">
      <span class="label">contrast:</span>
      <input id="contrast-select" type="range" min="25" max="150" value="45">
    </label>
  </menu>

  <h2>initial Light/Dark Based on User-OS</h2>
  <p>
    Each theme comes with different levels of customization.
    Since "triad" and "adjacent" themes 
    could go either way around the color wheel,
    we use one direction for the light mode,
    and the other for dark mode.
    Sass is only used to simplify the range-selector prefixes,
    and JS is only used to set custom properties.
    This is <a href="#">what a link looks like</a>.
  </p>

  <section data-colors="invert">
    <h2>data-colors='invert'</h2>
    <p>
      Lorem ipsum dolor sit amet, 
      <a href="#">consectetur adipisicing elit</a>. 
      Et, maiores itaque! Culpa natus officiis eos, 
      delectus libero impedit autem quae expedita tempora 
      ipsa excepturi numquam fugit facilis amet quibusdam 
      laudantium.
    </p>
  </section>

  <div class="palette" role="presentation">
    <h3 data-colors="invert">prime</h3>
    <div style="--show: var(--prime-palette)"></div>
  </div>
  
  <div class="palette" role="presentation">
    <h3 data-colors="invert">accent</h3>
    <div style="--show: var(--accent-palette)"></div>
  </div>
  
  <hr>

  <div class="palette" data-theme="complement" data-colors role="presentation">
    <h3 data-colors="invert">complement</h3>
    <div style="--show: var(--accent-palette)"></div>
  </div>

  <div class="palette" data-theme="triad" data-colors role="presentation">
    <h3 data-colors="invert">triad</h3>
    <div style="--show: var(--accent-palette)"></div>
  </div>

  <div class="palette" data-theme="adjacent" data-colors role="presentation">
    <h3 data-colors="invert">adjacent</h3>
    <div style="--show: var(--accent-palette)"></div>
  </div>
</main>
!

CSS 

              
                // primary hue
[data-theme] {
  // optionally set `--h-user-*` values with JS
  --h-prime: var(--h-user-prime, 330);
  --h-accent: var(--h-user-accent, 195);

  // initial lightness and saturation values
  --l-user-p: calc(var(--l-user) * 1%);
  --l: var(--l-user-p, 50%);
  --s-user-p: calc(var(--s-user) * 1%);
  --s: var(--s-user-p, 80%);

  // offset lightness and saturation by various amounts
  --offset-count: 3;
  --user-contrast-p: calc(var(--user-contrast) * 1%);
  --contrast: var(--user-contrast-p, 45%);

  --offset-step: calc(var(--contrast) / var(--offset-count));
  
  --off-1: calc(var(--offset-step));
  --off-2: calc(var(--offset-step) * 2);
  --off-3: var(--contrast);
  
  // generate faded values…
  --fade: 0.75;
}

// presets
[data-theme="default-pink"] {
  --h-prime: 330;
  --h-accent: 195;
  --custom-hue: none;
}

[data-theme="default-blue"] {
  --h-prime: 195;
  --h-accent: 330;
  --custom-hue: none;
}

[data-theme="black"] {
  --s: 0%;
  --l: 50%;
  --contrast: calc(100% * var(--offset-count));
  --custom-hue: none;
  --custom-sat: none;
  --custom-light: none;
  --custom-contrast: none;
}

// user customizable
[data-theme="monochrome"] {
  --h-accent: var(--h-prime);
}

[data-theme="complement"] {
  --h-accent: var(--h-complement);
}

[data-theme="triad"] {
  --h-accent: var(--h-triad);
}

[data-theme="adjacent"] {
  --h-accent: var(--h-adjacent);
}


// Light & Dark Modes
// ------------------

[data-colors] {
  @media (prefers-color-scheme: light) {
    --os-mode: 1;
  }

  @media (prefers-color-scheme: dark) {
    --os-mode: -1;
  }

  --given-mode: var(--user-mode, var(--html-mode, var(--os-mode, 1)));
  --mode: var(--given-mode);
  --invert: calc(-1 * var(--mode));
}

[data-colors="invert"] {
  --mode: calc(-1 * var(--given-mode));
}

[data-colors="light"] {
  --html-mode: 1;
}

[data-colors="dark"] {
  --html-mode: -1;
}


// Re-Calculate when mode changes
// ------------------------------

[data-colors] {  
  /* options for setting different color palettes */
  --h-complement: calc(var(--h-prime) + (180 * var(--mode)));
  --h-triad: calc(var(--h-prime) + (120 * var(--mode)));
  --h-adjacent: calc(var(--h-prime) + (60 * var(--mode)));
      
  /* full contrast colors, foreground and background */
  --bg-full: hsl(0, 0%, calc(100% * var(--mode)));
  --fg-full: hsl(0, 0%, calc(100% * var(--invert)));
  
  /* generate lightness values */
  --l-fg-1: calc(var(--l) - var(--off-1) * var(--mode));
  --l-fg-2: calc(var(--l) - var(--off-2) * var(--mode));
  --l-fg-3: calc(var(--l) - var(--off-3) * var(--mode));

  --l-bg-1: calc(var(--l) + var(--off-1) * var(--mode));
  --l-bg-2: calc(var(--l) + var(--off-2) * var(--mode));
  --l-bg-3: calc(var(--l) + var(--off-3) * var(--mode));
 
  /* generate saturation values */
  --s1: calc(var(--s) - var(--off-1));
  --s2: calc(var(--s) - var(--off-2));
  --s3: calc(var(--s) - var(--off-3));
  
  /* generate the prime color palette */
  --prime: hsl(var(--h-prime), var(--s), var(--l));
  --prime-fade: hsla(var(--h-prime), var(--s3), var(--l), var(--fade));
  --prime-fg-1: hsl(var(--h-prime), var(--s1), var(--l-fg-1));
  --prime-fg-1-fade: hsla(var(--h-prime), var(--s3), var(--l-fg-1), var(--fade));
  --prime-fg-2: hsl(var(--h-prime), var(--s2), var(--l-fg-2));
  --prime-fg-2-fade: hsla(var(--h-prime), var(--s3), var(--l-fg-2), var(--fade));
  --prime-fg-3: hsl(var(--h-prime), var(--s3), var(--l-fg-3));
  --prime-fg-3-fade: hsla(var(--h-prime), var(--s3), var(--l-fg-3), var(--fade));

  --prime-bg-1: hsl(var(--h-prime), var(--s1), var(--l-bg-1));
  --prime-bg-1-fade: hsla(var(--h-prime), var(--s3), var(--l-bg-1), var(--fade));
  --prime-bg-2: hsl(var(--h-prime), var(--s2), var(--l-bg-2));
  --prime-bg-2-fade: hsla(var(--h-prime), var(--s3), var(--l-bg-2), var(--fade));
  --prime-bg-3: hsl(var(--h-prime), var(--s3), var(--l-bg-3));
  --prime-bg-3-fade: hsla(var(--h-prime), var(--s3), var(--l-bg-3), var(--fade));

  /* generate the accent color palette */
  --accent: hsl(var(--h-accent), var(--s), var(--l));
  --accent-fade: hsla(var(--h-accent), var(--s3), var(--l), var(--fade));
  --accent-fg-1: hsl(var(--h-accent), var(--s1), var(--l-fg-1));
  --accent-fg-1-fade: hsla(var(--h-accent), var(--s3), var(--l-fg-1), var(--fade));
  --accent-fg-2: hsl(var(--h-accent), var(--s2), var(--l-fg-2));
  --accent-fg-2-fade: hsla(var(--h-accent), var(--s3), var(--l-fg-2), var(--fade));
  --accent-fg-3: hsl(var(--h-accent), var(--s3), var(--l-fg-3));
  --accent-fg-3-fade: hsla(var(--h-accent), var(--s3), var(--l-fg-3), var(--fade));

  --accent-bg-1: hsl(var(--h-accent), var(--s1), var(--l-bg-1));
  --accent-bg-1-fade: hsla(var(--h-accent), var(--s3), var(--l-bg-1), var(--fade));
  --accent-bg-2: hsl(var(--h-accent), var(--s2), var(--l-bg-2));
  --accent-bg-2-fade: hsla(var(--h-accent), var(--s3), var(--l-bg-2), var(--fade));
  --accent-bg-3: hsl(var(--h-accent), var(--s3), var(--l-bg-3));
  --accent-bg-3-fade: hsla(var(--h-accent), var(--s3), var(--l-bg-3), var(--fade));

  /* flip the shadow based on the mode */
  --shadow-offset: calc(1px * var(--mode));

  /* color props set directly by the theme */
  background: var(--accent-bg-3);
  color: var(--prime-fg-3);
  text-shadow: var(--shadow-offset) var(--shadow-offset) 0 var(--bg-full);
}


/* some UI elements… */
a {
  --link-color: var(--prime-fg-2);
  --link-underline: var(--prime-fg-2);
  color: var(--link-color);
  text-decoration-color: var(--link-underline);
  text-decoration-skip-ink: auto;

  &:hover,
  &:focus,
  &:active {
    --link-color: var(--prime-fg-3);
    --link-underline: var(--prime);  
  }
}

button {
  --button-background: var(--accent-fg-2);  
  --button-color: var(--accent-bg-3);
  --button-border: var(--accent-fg-3-fade);
  background: var(--button-background);
  border: 2px solid var(--button-border);
  color: var(--button-color);
  cursor: pointer;
  padding: 0.25em 0.5em;

  &:hover,
  &:focus {
    --button-background: var(--accent-fg-3);  
    --button-border: var(--accent-fg-3);
  }
}

button:active {
  --button-background: var(--prime-bg-2);    
  --button-color: var(--prime-fg-2);
}


/* show the colors */
.palette {
  background: linear-gradient(
    45deg,
    var(--accent-fg-3) 25%,
    transparent 25% 75%,
    var(--accent-fg-3) 75%
  ), 
  linear-gradient(
    45deg,
    var(--accent-fg-3) 25%,
    transparent 25% 75%,
    var(--accent-fg-3) 75%
  )
  var(--accent-bg-3);
  background-size: 1em 1em;
  background-position: 0 0, 0.5em 0.5em;
  border: 2px solid var(--fg-full);
  margin: 1em 0;
  
  h3 {
    font-size: 0.75em;
    padding: 0.25em 1em;
  }
}

[style*="--show"] {
  --stop: calc(100% / (var(--offset-count) * 2 + 1));
  --accent-palette-main: linear-gradient(to right,
    var(--accent-bg-3) calc(var(--stop) * 1),
    var(--accent-bg-2) calc(var(--stop) * 1) calc(var(--stop) * 2),
    var(--accent-bg-1) calc(var(--stop) * 2) calc(var(--stop) * 3),
    var(--accent) calc(var(--stop) * 3) calc(var(--stop) * 4),
    var(--accent-fg-1) calc(var(--stop) * 4) calc(var(--stop) * 5),
    var(--accent-fg-2) calc(var(--stop) * 5) calc(var(--stop) * 6),
    var(--accent-fg-3) calc(var(--stop) * 6)
  );
  --accent-palette-fade: linear-gradient(to right,
    var(--accent-bg-3-fade) calc(var(--stop) * 1),
    var(--accent-bg-2-fade) calc(var(--stop) * 1) calc(var(--stop) * 2),
    var(--accent-bg-1-fade) calc(var(--stop) * 2) calc(var(--stop) * 3),
    var(--accent-fade) calc(var(--stop) * 3) calc(var(--stop) * 4),
    var(--accent-fg-1-fade) calc(var(--stop) * 4) calc(var(--stop) * 5),
    var(--accent-fg-2-fade) calc(var(--stop) * 5) calc(var(--stop) * 6),
    var(--accent-fg-3-fade) calc(var(--stop) * 6)
  );
  --prime-palette-main: linear-gradient(to right,
    var(--prime-bg-3) calc(var(--stop) * 1),
    var(--prime-bg-2) calc(var(--stop) * 1) calc(var(--stop) * 2),
    var(--prime-bg-1) calc(var(--stop) * 2) calc(var(--stop) * 3),
    var(--prime) calc(var(--stop) * 3) calc(var(--stop) * 4),
    var(--prime-fg-1) calc(var(--stop) * 4) calc(var(--stop) * 5),
    var(--prime-fg-2) calc(var(--stop) * 5) calc(var(--stop) * 6),
    var(--prime-fg-3) calc(var(--stop) * 6)
  );
  --prime-palette-fade: linear-gradient(to right,
    var(--prime-bg-3-fade) calc(var(--stop) * 1),
    var(--prime-bg-2-fade) calc(var(--stop) * 1) calc(var(--stop) * 2),
    var(--prime-bg-1-fade) calc(var(--stop) * 2) calc(var(--stop) * 3),
    var(--prime-fade) calc(var(--stop) * 3) calc(var(--stop) * 4),
    var(--prime-fg-1-fade) calc(var(--stop) * 4) calc(var(--stop) * 5),
    var(--prime-fg-2-fade) calc(var(--stop) * 5) calc(var(--stop) * 6),
    var(--prime-fg-3-fade) calc(var(--stop) * 6)
  );
  --prime-palette: var(--prime-palette-main), var(--prime-palette-fade);
  --accent-palette: var(--accent-palette-main), var(--accent-palette-fade);
  background-image: var(--show);
  background-size: 100% 50%, 100% 100%;
  background-repeat: no-repeat;
  height: 3em;
}


/* layout */
body {
  display: grid;
  min-height: 100vh;
  font-size: calc(1em + 0.5vmin);
}

main {
  --growth: 3vw;
  padding: 0.5em calc(0.25em + var(--growth));
  transition: padding 100ms ease-out;
  
  @media (min-width: 45em) {
    --growth: 10vw;
  }

  @media (min-width: 80em) {
    --growth: 15vw;
  }
}

section {
  padding: 0.5em 1em;
}

menu {
  align-items: center;
  display: grid;
  grid-auto-rows: 1fr;
  grid-template-columns: repeat(4, 1fr);
  grid-gap: 0.5em 1.5em;
  font-size: smaller;
  margin-bottom: 2em;
  padding: 0;
  
  @media (min-width: 45em) {
    --min: 18em;
  }

  @media (min-width: 70em) {
    --min: 20em;
  }
}

[data-toggle] {
  justify-self: end;
  grid-column: span 1 / -1;
}

[for*='select']  {
  grid-column: span 2;
}

[for='theme-select'] {
  grid-column: span 3;
}

.label {
  display: block;
}

[for='hue-select'] {
  display: var(--custom-hue, block);
}

[for='light-select'] {
  display: var(--custom-light, block);
}

[for='sat-select'] {
  display: var(--custom-sat, block);
}

[for='contrast-select'] {
  display: var(--custom-contrast, block);
}

@supports (background-clip: text) {
  h1 {
    background: linear-gradient(to bottom right, var(--prime-fg-1), var(--accent-fg-1));
    background-clip: text;
    box-decoration-break: clone;
    color: transparent;
    text-shadow: none;
  }
}

h2 {
  font-size: 1em;
}

h3, h2 {
  margin: 0;
}

::selection {
  background: var(--prime-fg-2);
  color: var(--prime-bg-3);
  background-clip: initial;
  text-shadow: none;
}


/* range input */
@mixin range-track {
  &::-webkit-slider-runnable-track { @content; }
  &::-moz-range-track { @content; }
  &::-ms-track { @content; }
}

@mixin range-thumb {
  &::-webkit-slider-thumb { @content; }
  &::-moz-range-thumb { @content; }
  &::-ms-thumb { @content; }
}

$rainbow: ();
@for $hue from 1 through 36 {
  $rainbow: append($rainbow, hsl($hue * 10, 80%, 40%), comma);
}

[type='range'] {
  -webkit-appearance: none;
  background: transparent;
  height: 0.75em;
  width: 100%;
  
  @include range-track {
    height: 1em;
    background: var(--accent-fg-3-fade);
    border-radius: 1em;
    border: 1px solid var(--accent-fg-2-fade);
    cursor: pointer;
  }
  
  @include range-thumb {
    border: 2px solid var(--accent-fg-3);
    height: 1em;
    width: 0.5em;
    border-radius: 0.25em;
    background: var(--prime);
    cursor: pointer;
  }
}

[id='hue-select'] {
  @include range-track {
    background: linear-gradient(
      to right,
      $rainbow
    );
  }
}

[id='light-select'] {
  @include range-track {
    background: linear-gradient(
      to right,
      hsl(0, 0%, 25%),
      hsl(0, 0%, 75%)
    );
  }
}

[id='sat-select'] {
  @include range-track {
    background: linear-gradient(
      to right,
      hsl(var(--h-prime), 0%, var(--l)),
      hsl(var(--h-prime), 100%, var(--l))
    );
  }
}

[id='contrast-select'] {
  @include range-track {
    background: linear-gradient(
      to right,
      hsl(0, 0%, 50%),
      hsl(0, 0%, 100%)
    ),
    linear-gradient(
      to right,
      hsl(0, 0%, 50%),
      hsl(0, 0%, 0%)
    );
    background-size: 100% 50%, 100% 100%;
    background-position: top left, bottom left;
    background-repeat: no-repeat;
  }
}
!

JS 

              
                const themeAttr = 'data-theme';

const root = document.querySelector(`[${themeAttr}]`);
const modeToggle = document.querySelector('[data-toggle]');

const hueProp = '--h-user-prime';
const satProp = '--s-user';
const lightProp = '--l-user';
const contrastProp = '--user-contrast';
const modeProp = '--user-mode';

const themeSelect = document.querySelector('#theme-select');
const hueSelect = document.querySelector('#hue-select');
const satSelect = document.querySelector('#sat-select');
const lightSelect = document.querySelector('#light-select');
const contrastSelect = document.querySelector('#contrast-select');

const changeMode = () => {
  const is = getComputedStyle(root)
    .getPropertyValue('--mode')
    .trim();
  changeProp(modeProp, Number(is) * -1);
}

const changePreset = () => {
  const to = themeSelect.value;
  root.setAttribute(themeAttr, to);
}

const changeProp = (prop, val) => {
  root.style.setProperty(prop, val);
}

document.onload = changePreset();
themeSelect.addEventListener('input', () => changePreset());
modeToggle.addEventListener('click', () => changeMode());

hueSelect.addEventListener('input', () => changeProp(hueProp, hueSelect.value));
satSelect.addEventListener('input', () => changeProp(satProp, satSelect.value));
lightSelect.addEventListener('input', () => changeProp(lightProp, lightSelect.value));
contrastSelect.addEventListener('input', () => changeProp(contrastProp, contrastSelect.value));
!
999px

Console