<main>
  <div>
    <label for="lightdark">Light/Dark Mode:</label>
    <select id="lightdark">
      <option value="auto">Auto</option>
      <option value="light">Light</option>
      <option value="dark">Dark</option>
    </select>
  </div>
  <div>
    <label for="contrast">Contrast Mode:</label>
    <select id="contrast">
      <option value="auto">Auto</option>
      <option value="high">High</option>
      <option value="low">Low</option>
    </select>
  </div>
</main>

:root {
  /* allow unstyled HTML to follow user preference via prefers-color-scheme */
  color-scheme: light dark;
}

:root:has(#lightdark option[value="light"]:checked) {
  color-scheme: light;
}
:root:has(#lightdark option[value="dark"]:checked) {
  color-scheme: dark;
}
:root:has(#contrast option[value="low"]:checked) {
  filter: contrast(75%);
  /* only needed because of low contrast filter, which needs a declared color to work on: */
  color: CanvasText;
  background-color: Canvas;
}
:root:has(#contrast option[value="high"]:checked) {
  & main {
    background-color: Canvas;
    border: 2px solid CanvasText;
  }
}

@media (prefers-contrast: less) {
  :root {
    filter: contrast(75%);
    /* only needed because of low contrast filter, which needs a declared color to work on: */
    color: CanvasText;
    background-color: Canvas;
  }
  :root:has(#contrast option[value="high"]:checked) {
    filter: revert;
  }
}

/* just prettifying */
body {
  display: grid;
  place-content: center;
  min-height: 100svh;
  font-family: system-ui, sans-serif;
}
main {
  display: grid;
  gap: 1em;
  padding: 1em;
  border-radius: 0.75em;
  background-color: color-mix(in srgb, rebeccapurple 40%, Canvas);
}
div {
  display: flex;
  justify-content: space-between;
  gap: 1em;
}