<div class="wrapper">
  <div class="element">
    <div class="code">rgb(50 50 50 / .5)</div>
  </div>
</div>

<div class="form">
  <div class="control">
    <label for="red">R(Red):<output class="output" for="red">50</output></label>
    <div class="rang">
      <span class="min">0</span>
      <input type="range" id="red" value="50" min="0" max="255" step="1" class="range" />
      <span class="max">255</span>
    </div>
  </div>

  <div class="control">
    <label for="green">G(Green):<output class="output" for="green">50</output></label>
    <div class="rang">
      <span class="min">0</span>
      <input type="range" id="green" value="50" min="0" max="255" step="1" class="range" />
      <span class="max">255</span>
    </div>
  </div>

  <div class="control">
    <label for="blue">B(Blue):<output class="output" for="blue">50</output></label>
    <div class="rang">
      <span class="min">0</span>
      <input type="range" id="blue" value="50" min="0" max="255" step="1" class="range" />
      <span class="max">255</span>
    </div>
  </div>

  <div class="control">
    <label for="alpha">A(Alpha):<output class="output" for="alpha">.5</output></label>
    <div class="rang">
      <span class="min">0</span>
      <input type="range" id="alpha" value=".5" min="0" max="1" step="0.1" class="range" />
      <span class="max">1</span>
    </div>
  </div>
</div>

@import url("https://fonts.googleapis.com/css?family=Gochi+Hand");

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

body {
  margin: 0;
  padding: 0;
  background-color: #291642;
  font-family: "Gochi Hand", sans-serif;
  color: #fff;
  font-size: 130%;
  letter-spacing: 0.1rem;
  display: flex;
  align-items: center;
  justify-content: center;
  flex-direction: column;
  width: 100vw;
  min-height: 100vh;
  padding: 10px;
}

.form {
  padding: 20px;
  width: 100%;
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 10px;
}

label {
  display: block;
  margin-bottom: 5px;
  white-space: nowrap;
}

.control {
  margin: 20px 10px;
}

.c__box {
  grid-column: span 3;
  text-align: center;
  white-space: nowrap;
}

.rang {
  display: grid;
  grid-template-columns: auto 1fr auto;
  gap: 5px;
}
.min {
  text-align: right;
}

.output {
  text-align: center;
  border: 1px dashed #f36;
  margin-left: 4px;
  border-radius: 5px;
  padding: 2px 6px;
  position: relative;
}

.checkbox {
  margin-right: 5px;
}

.wrapper {
  flex: 1;
  display: flex;
  justify-content: center;
  align-items: center;
  position: relative;
}

.select {
  margin-bottom: 10px;
  display: flex;
  align-items: center;
  justify-content: center;
}

.select .control {
  display: flex;
  align-items: center;
  align-items: center;
}
.select label {
  white-space: nowrap;
}

.element {
  width: 40vw;
  min-height: 30vh;
  border-radius: 5px;
  border-radius: 10px;
  padding: 30px;
  border: 1px dashed currentColor;
  display: flex;
  justify-content: center;
  align-items: center;
}

:root {
  --red: 50;
  --green: 50;
  --blue: 50;
  --alpha: .5;
}

.code {
  font-size: 2rem;
  color: rgb(var(--red) var(--green) var(--blue) / var(--alpha));
}

const ranges = document.querySelectorAll(".range");
const code = document.querySelector(".code");

const arr = [...ranges].map((range, index) => {
  range.addEventListener("change", function (e) {
    handleUpdate(e, index);
  });
  return range.value;
});

function handleUpdate(e, index) {
  e.target.parentNode.parentNode.querySelector(
    "output"
  ).textContent = `${e.target.value}`;

  arr[index] = e.target.value;

  document.documentElement.style.setProperty(
    `--${e.target.id}`,
    `${e.target.value}`
  );

  code.textContent = `rgb(${arr[0]} ${arr[1]} ${arr[2]} / ${arr[3]})`;
}