<body>
  <h1>Theme Color Picker</h1>
  <p>Try to change the color from the color picker on the corner.</p>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
  <input id="colorPicker" type="color" />
</body>
@import url('https://fonts.googleapis.com/css2?family=Roboto&display=swap');

body{
  text-align:center;
  padding: 50px;
  font-family: 'Roboto', sans-serif;
}

input{
  width: 50px;
  height: 50px;
  outline: none;
  border:none;
  position: absolute;
  top: 10px;
  left: 10px;
  cursor: pointer;
}
//Get the color picker
const colorpicker = document.getElementById("colorPicker");
//Change colors when the color picker value changes
colorpicker.addEventListener("change", ChangeColor)

function ChangeColor (){
    //Remove the #
    const mainColor = colorpicker.value.slice(1);
    //Get the complementary color
    const complementaryColor = invertHex(mainColor);

    //Apply new colors
    document.body.style.color = "#"+mainColor;
    document.body.style.background = "#"+complementaryColor;
}

function invertHex(hex) {
    return (Number(`0x1${hex}`) ^ 0xCCCCCC).toString(16).substr(1).toUpperCase()
  }
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