<div class="control">
  <label for="angle">Angle:</label>
  <input type="range" id="angle" name="angle" value="0" min="0" max="360" step="1" />
  <output id="output">0deg</output>
</div>

<div class="control">
  <label for="position">position:</label>
  <select name="position" id="position">
    <optgroup label="bg-position">
      <option value="top">top</option>
      <option value="right">right</option>
      <option value="bottom">bottom</option>
      <option value="left">left</option>
      <option value="center" selected>center</option>
      <option value="top left">top left</option>
      <option value="top center">top center</option>
      <option value="top right">top right</option>
      <option value="center center">center center</option>
      <option value="center right">center right</option>
      <option value="left center">let center</option>
      <option value="bottom left">bottom left</option>
      <option value="bottom center">bottom center</option>
      <option value="bottom right">bottom right</option>
    </optgroup>
    <optgroup label="length-percentage">
      <option value="10px 50px">10px 50px</option>
      <option value="10em 50em">10em 50em</option>
      <option value="10rem 50rem">10rem 50rem</option>
      <option value="10vw 50vw">10vw 50vw</option>
      <option value="10vh 50vh">10vh 50vh</option>
      <option value="10% 50%">10% 50%</option>
      <option value="10%">10%</option>
      <option value="10px 50%">10px 50%</option>
      <option value="10% 50px">10% 50px</option>
    </optgroup>
  </select>
</div>
@import url("https://fonts.googleapis.com/css2?family=Exo:wght@600&display=swap");

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  width: 100vw;
  min-height: 100vh;
  font-family: "Exo", Arial, sans-serif;
  background-color: #222;
  color: #fff;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.5rem;
}

:root {
  --angle: 0deg;
  --position: center;
}
body {
  background-image: conic-gradient(
    from var(--angle) at var(--position),
    #09f,
    #f36
  );
}

.control {
  margin: 0 5px;
}
const rootElement = document.documentElement;
const angleInput = document.getElementById("angle");
const angleOutput = document.getElementById("output");
const position = document.getElementById("position");

angleInput.addEventListener("input", (etv) => {
  rootElement.style.setProperty("--angle", `${etv.target.value}deg`);
  angleOutput.innerText = `${etv.target.value}deg`;
});

position.addEventListener("change", (etv) => {
  rootElement.style.setProperty("--position", etv.target.value);
});
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