HTMLCSSJS

<button class="theme-toggle" id="theme-toggle" title="Toggles light & dark" aria-label="auto" aria-live="polite">
  <svg class="sun-and-moon" aria-hidden="true" width="24" height="24" viewBox="0 0 24 24">
    <mask class="moon" id="moon-mask">
      <rect x="0" y="0" width="100%" height="100%" fill="white" />
      <circle cx="24" cy="10" r="6" fill="black" />
    </mask>
    <circle class="sun" cx="12" cy="12" r="6" mask="url(#moon-mask)" fill="currentColor" />
    <g class="sun-beams" stroke="currentColor">
      <line x1="12" y1="1" x2="12" y2="3" />
      <line x1="12" y1="21" x2="12" y2="23" />
      <line x1="4.22" y1="4.22" x2="5.64" y2="5.64" />
      <line x1="18.36" y1="18.36" x2="19.78" y2="19.78" />
      <line x1="1" y1="12" x2="3" y2="12" />
      <line x1="21" y1="12" x2="23" y2="12" />
      <line x1="4.22" y1="19.78" x2="5.64" y2="18.36" />
      <line x1="18.36" y1="5.64" x2="19.78" y2="4.22" />
     </g>
  </svg>
</button>

        @import "https://unpkg.com/open-props/easings.min.css";

.sun-and-moon > :is(.moon, .sun, .sun-beams) {
  transform-origin: center;
}

.sun-and-moon > :is(.moon, .sun) {
  fill: var(--icon-fill);
}

.theme-toggle:is(:hover, :focus-visible) > .sun-and-moon > :is(.moon, .sun) {
  fill: var(--icon-fill-hover);
}

.sun-and-moon > .sun-beams {
  stroke: var(--icon-fill);
  stroke-width: 2px;
}

.theme-toggle:is(:hover, :focus-visible) .sun-and-moon > .sun-beams {
  stroke: var(--icon-fill-hover);
}

[data-theme="dark"] .sun-and-moon > .sun {
  transform: scale(1.75);
}

[data-theme="dark"] .sun-and-moon > .sun-beams {
  opacity: 0;
}

[data-theme="dark"] .sun-and-moon > .moon > circle {
  transform: translateX(-7px);
}

@supports (cx: 1) {
  [data-theme="dark"] .sun-and-moon > .moon > circle {
    cx: 17;
    transform: translateX(0);
  }
}

@media (prefers-reduced-motion: no-preference) {
  .sun-and-moon > .sun {
    transition: transform .5s var(--ease-elastic-3);
  }

  .sun-and-moon > .sun-beams {
    transition: transform .5s var(--ease-elastic-4), opacity .5s var(--ease-3);
  }

  .sun-and-moon .moon > circle {
    transition: transform .25s var(--ease-out-5);
  }

  @supports (cx: 1) {
    .sun-and-moon .moon > circle {
      transition: cx .25s var(--ease-out-5);
    }
  }

  [data-theme="dark"] .sun-and-moon > .sun {
    transition-timing-function: var(--ease-3);
    transition-duration: .25s;
    transform: scale(1.75);
  }

  [data-theme="dark"] .sun-and-moon > .sun-beams {
    transition-duration: .15s;
    transform: rotateZ(-25deg);
  }

  [data-theme="dark"] .sun-and-moon > .moon > circle {
    transition-duration: .5s;
    transition-delay: .25s;
  }
}
        

        const storageKey = 'theme-preference'

const onClick = () => {
  // flip current value
  theme.value = theme.value === 'light'
    ? 'dark'
    : 'light'

  setPreference()
}

const getColorPreference = () => {
  if (localStorage.getItem(storageKey))
    return localStorage.getItem(storageKey)
  else
    return window.matchMedia('(prefers-color-scheme: dark)').matches
      ? 'dark'
      : 'light'
}

const setPreference = () => {
  localStorage.setItem(storageKey, theme.value)
  reflectPreference()
}

const reflectPreference = () => {
  document.firstElementChild
    .setAttribute('data-theme', theme.value)

  document
    .querySelector('#theme-toggle')
    ?.setAttribute('aria-label', theme.value)
}

const theme = {
  value: getColorPreference(),
}

// set early so no page flashes / CSS is made aware
reflectPreference()

window.onload = () => {
  // set on load so screen readers can see latest value on the button
  reflectPreference()

  // now this script can find and listen for clicks on the control
  document
    .querySelector('#theme-toggle')
    .addEventListener('click', onClick)
}

// sync with system changes
window
  .matchMedia('(prefers-color-scheme: dark)')
  .addEventListener('change', ({matches:isDark}) => {
    theme.value = isDark ? 'dark' : 'light'
    setPreference()
  })