<input class="dark" type="checkbox" />
<input class="light" type="checkbox" />
<article>
  <h1>Dark Mode w Pure CSS</h1>
  <p>
    This is an example to display how to support dark mode without using
    any Javascript.
  </p>
</article>

:root {
  --bg-color: #ffffff75;
  --font-color: #121212;
  --em-font-color: #121212;
  --border-color: #218095;
  --gradient-style: radial-gradient(
    circle,
    #f6ff1c,
    #ffdc00,
    #ffb800,
    #ff951a
  );
  --gradient-size: 8px;
  --gradient-shadow: #ffb800;
}
@media (prefers-color-scheme: light) {
  .light {
    display: none;
  }
}
@media (prefers-color-scheme: dark) {
  .dark {
    display: none;
  }
  --bg-color: #121212;
  --font-color: #ffffff75;
  --em-font-color: #ffffff87;
  --border-color: #c9e3e9;
  --gradient-style: radial-gradient(
    circle,
    #ccd325,
    #d3d84b,
    #dadd68,
    #e0e183,
    #e6e69c
  );
  --gradient-size: 2px;
  --gradient-shadow: #fff;
}

/* swith to dark mode when dark button checked */
.dark:checked,
.dark:checked + * + * {
  --bg-color: #121212;
  --font-color: #ffffff75;
  --em-font-color: #ffffff87;
  --border-color: #c9e3e9;
  --gradient-style: radial-gradient(
    circle,
    #ccd325,
    #d3d84b,
    #dadd68,
    #e0e183,
    #e6e69c
  );
  --gradient-size: 2px;
  --gradient-shadow: #fff;
}

/* swith to light mode when light button checked */
.light:checked,
.light:checked + * {
  --bg-color: #ffffff75;
  --font-color: #121212;
  --em-font-color: #121212;
  --border-color: #218095;
  --gradient-style: radial-gradient(
    circle,
    #f6ff1c,
    #ffdc00,
    #ffb800,
    #ff951a
  );
  --gradient-size: 8px;
  --gradient-shadow: #ffb800;
}

/* apply css variable */
body {
  margin: 0;
}
* {
  background: var(--bg-color);
  color: var(--font-color);
}

input[type="checkbox"] {
  -webkit-appearance: none;
  outline: 0;
  position: absolute;
  z-index: 2;
  top: 10px;
  right: 10px;
  width: 20px;
  height: 20px;
  border-radius: 50%;
  background-image: var(--gradient-style);
  box-shadow: 0 0 14px var(--gradient-size) var(--gradient-shadow);
  cursor: pointer;
}
article {
  position: absolute;
  width: 100vw;
  height: 100vh;
  padding: 24px;
}
h1 {
  font-size: 24px;
  margin-top: 20px;
  padding-bottom: 4px;
  border-bottom: 3px solid var(--border-color);
  color: var(--em-font-color);
}
p {
  font-size: 18px;
  margin: 10px 0;
  line-height: 1.4;
}