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HTML 

              
                <body id="body" onkeydown="return myKeyPress(event)" class="light-mode" data="dark-mode">
  <div class="container">
    <div class="mode-btn"> <button type="button" onclick="change_mode()" class="mode-change" id="mode-change">Change
        mode</button></div>
    <table class="main-box" cellspacing=0 cellpadding=0>
      <thead class="display-box">
        <tr>
          <th colspan="5">
            <div class="calc-display" id="display"></div>
          </th>
        </tr>
        <tr>
          <th colspan="5">
            <div class="calc-display" id="solutions"></div>
          </th>
        </tr>
      </thead>
      <tbody class="inner-box">
        <tr>
          <td>
            <button class=" " id="clear">C</button>
          </td>
          <td>
            <button class=" dark-btn" id="sign">+/-</button>
          </td>
          <td>
            <button class=" dark-btn operator" id="percent">%</button>
          </td>
          <td>
            <button class=" blue-btn operator">÷</button>
          </td>
        </tr>
        <tr>
          <td>
            <button class="  num">7</button>
          </td>
          <td>
            <button class="  num">8</button>
          </td>
          <td>
            <button class="  num">9</button>
          </td>
          <td>
            <button class=" blue-btn operator">x</button>
          </td>
        </tr>
        <tr>
          <td>
            <button class="  num">4</button>
          </td>
          <td>
            <button class="  num">5</button>
          </td>
          <td>
            <button class="  num">6</button>
          </td>
          <td>
            <button class=" blue-btn  operator">-</button>
          </td>
        </tr>
        <tr>
          <td>
            <button class="  num">1</button>
          </td>
          <td>
            <button class="  num">2</button>
          </td>
          <td>
            <button class="  num">3</button>
          </td>
          <td>
            <button class=" blue-btn operator">+</button>
          </td>
        </tr>
        <tr>
          <td>
            <button class=" num" id="zero-button">0</button>
          </td>
          <td>
            <button class=" num" id="zero-button">00</button>
          </td>
          <td>
            <button class="  num" id="decimal-point">.</button>
          </td>
          <td>
            <button class=" " id="equal">=</button>
          </td>
        </tr>
      </tbody>
    </table>
  </div>
</body>
!

CSS 

              
                * {
  margin: 0;
  padding: 0;
}
body {
  margin: 0;
  padding: 0;
  height: 100vh;
  box-sizing: border-box;
  display: grid;
  place-items: center;
  font-family: "Roboto", sans-serif;
  font-weight: 400;
}
body.dark-mode {
  background-color: #1f1f1f;
}

#display,
#solutions {
  margin: 0;
  height: auto;
  min-height: 80px;
  width: 100%;
  max-width: 100%;
  box-sizing: border-box;
  border: none;
  outline: none;
  color: #747474;
  font-size: 46px;
  line-height: 60px;
  font-weight: 600;
  padding: 5px 15px;
  word-wrap: break-word;
  word-break: break-all;
  text-align: right;
  font-weight: 300;
  overflow: hidden;
}
#solutions {
  font-weight: 400;
  font-size: 32px;
  line-height: 46px;
}
.dark-mode #display {
  color: #9e9e9e;
}
.dark-mode #solutions {
  color: #dbdbdb;
}
button {
  margin: 10px;
  width: 80px;
  height: 80px;
  font-size: 36px;
  line-height: 50px;
  border-radius: 50%;
  cursor: pointer;
  border: none;
  color: #747474;
  outline: none;
  display: flex;
  justify-content: center;
  align-items: center;
}
.light-mode button {
  background-color: #f1f3f6;
  box-shadow: -6px -6px 20px 0 #ffffff, 6px 6px 20px 0 rgba(0, 0, 0, 0.08),
    inset 0 1px 3px 0 rgba(0, 0, 0, 0.05);
}
.light-mode button:hover,
.light-mode button:focus {
  box-shadow: inset 4px 4px 10px 0 rgba(0, 0, 0, 0.08),
    inset -4px -4px 10px 0 #fff;
}
.dark-mode button {
  background-color: #1f1f1f;
  box-shadow: -6px -6px 20px 0 #2b2b2b, 6px 6px 20px 0 rgba(0, 0, 0, 0.08),
    inset 0 1px 3px 0 #000000;
}
.dark-mode button:hover,
.dark-mode button:focus {
  box-shadow: inset 4px 4px 10px 0 rgba(0, 0, 0, 0.08),
    inset -4px -4px 10px 0 #2b2b2b;
}
.dark-mode button {
  background-color: #1f1f1f;
  box-shadow: -6px -6px 20px 0 #000000, 6px 6px 20px 0 rgba(0, 0, 0, 0.08),
    inset 0 1px 3px 0 rgba(0, 0, 0, 0.05);
}
.dark-mode button:hover,
.dark-mode button:focus {
  box-shadow: inset 4px 4px 10px 0 rgba(0, 0, 0, 0.08),
    inset -4px -4px 10px 0 #000;
}
td {
  max-width: 100px;
}
#equal {
  background-color: #ffbd2e;
  color: white;
}
.main-box {
  border-collapse: collapse;
  border-radius: 20px;
  overflow: hidden;
  max-width: 425px;
}
.inner-box {
  margin: 4px;
}
.light-mode button.dark-btn {
  background-color: #dedede;
}
.light-mode button.dark-btn:hover,
.light-mode button.dark-btn:focus {
  box-shadow: inset 4px 4px 10px 0 rgba(0, 0, 0, 0.08),
    inset -4px -4px 10px 0 #dedede;
}
button.blue-btn {
  background-color: #6f9bdc;
  color: #fff;
}
button.blue-btn:hover,
button.blue-btn:focus {
  box-shadow: inset 4px 4px 10px 0 rgba(0, 0, 0, 0.08),
    inset -4px -4px 10px 0 #6f9bdc;
}
button#equal:hover,
button#equal:focus {
  box-shadow: inset 4px 4px 10px 0 rgba(0, 0, 0, 0.08),
    inset -4px -4px 10px 0 #ffbd2e;
}
.light-mode .main-box {
  background-color: #f1f3f6;
  box-shadow: -3px -3px 10px 0 #fff, 3px 3px 10px rgba(0, 0, 0, 0.08),
    inset 0 1px 3px rgba(0, 0, 0, 0.05);
  border: 12px solid #f1f3f6;
}
.dark-mode .main-box {
  background-color: #1f1f1f;
  box-shadow: -3px -3px 10px 0 #323232, 3px 3px 10px rgba(0, 0, 0, 0.8),
    inset 0 1px 3px rgba(0, 0, 0, 0.5);
  border: 12px solid #1f1f1f;
}
.dark-mode button.dark-btn {
  background-color: #000000;
}
.dark-mode button.dark-btn:hover,
.dark-mode button.dark-btn:focus {
  box-shadow: inset 4px 4px 10px 0 rgba(0, 0, 0, 0.08),
    inset -4px -4px 10px 0 #000000;
}
button.mode-change {
  padding: 12px 30px;
  width: auto !important;
  height: auto !important;
  border-radius: 12px;
  font-size: 20px;
  line-height: 28px;

  box-shadow: none !important;
  text-transform: capitalize;
  letter-spacing: 2px;
  display: block;
  margin: 15px 0 15px auto !important;
}
.light-mode button.mode-change {
  color: #fff;
  background-color: #1f1f1f;
}
.dark-mode button.mode-change {
  background-color: #fff;
  color: #1f1f1f;
}
.mode-btn {
  text-align: right;
}
.light-mode button.mode-change:hover {
  background-color: #2b2b2b;
}
.dark-mode button.mode-change:hover {
  color: #1f1f1f;
}
/* responsive css */
@media (max-width: 767px) {
  button {
    margin: 8px;
    width: 50px;
    height: 50px;
    font-size: 20px;
    line-height: 30px;
  }
  button.mode-change {
    padding: 10px 26px;
    font-size: 16px;
    line-height: 24px;
  }
}
!

JS 

              
                function change_mode() {
  var element = document.getElementById("body");
  element.classList.toggle("light-mode");
  element.classList.toggle("dark-mode");
}
/*  document.getElementById('display').innerHTML = "";
    document.getElementById('solutions').innerHTML = ""; */
const display = document.querySelector("#display");
const solutions = document.querySelector("#solutions");
const clearBtn = document.querySelector("#clear");
const signBtn = document.querySelector("#sign");
const percentageBtn = document.querySelector("#percent");
const equalBtn = document.querySelector("#equal");
const operators = ["+", "-", "*", "/", "%"];
let numberString = "";
let equation = "";
let result = null;
let operator = "";
// event listener for when a number is clicked
document.addEventListener("click", (e) => {
  if (!e.target.matches(".num")) return;
  // if the number string is "", start a new number and remove the display text
  if (numberString === "") {
    display.innerText = "";
  }
  // if result is not null, clear the calculation
  if (result != null) {
    clear();
  }
  // get the button value, add it to the display and the number string
  const value = e.target.innerText;
  if (value === ".") {
    checkDecimal();
    return;
  }
  display.innerText += value;
  numberString = display.innerText;
});
// event listener for when an operator is clicked
document.addEventListener("click", (e) => {
  if (!e.target.matches(".operator")) return;
  // if the result is not null, add the result to the equation, else add the number string
  if (result != null) {
    equation = result.toString();
    solutions.innerText = equation;
    result = null;
  } else {
    equation += numberString;
  }
  // get the operator
  operator = e.target.innerText;
  if (operator === "x") {
    operator = "*";
  }
  if (operator === "÷") {
    operator = "/";
  }
  /* numberString = "" */
  checkOperator();
});
// event listener for the +/- button
signBtn.addEventListener("click", () => {
  const convertDisplay = parseFloat(display.innerText);
  // if the result is not null, use the result and add or remove "-"
  if (result !== null) {
    equation = "";
    if (convertDisplay > 0) {
      display.innerText = "-" + result.toString();
      numberString = display.innerText;
    } else if (convertDisplay < 0) {
      const string = result.toString();
      const negativeSign = display.innerText.substr(string[0], 1);
      display.innerText = string.replace(negativeSign, "");
      numberString = display.innerText;
    }
    result = null;
    return;
  }
  // if the result is null, use the currently displayed text
  if (convertDisplay > 0) {
    display.innerText = "-" + display.innerText;
    numberString = display.innerText;
  } else if (convertDisplay < 0) {
    const negativeSign = display.innerText.substr(display.innerText[0], 1);
    display.innerText = display.innerText.replace(negativeSign, "");
    numberString = display.innerText;
  }
});
// event listener for the clear button
clearBtn.addEventListener("click", clear);
// event listener for equals button
equalBtn.addEventListener("click", () => {
  /*  equation += numberString 
       result = eval(equation) */
  result = eval(numberString);
  solutions.innerText = result;
});
// do not allow two decimal points in a row
function checkDecimal() {
  const lastChar = numberString.charAt(numberString.length - 1);
  if (lastChar === ".") return;
  else {
    numberString += ".";
    solutions.innerText = numberString;
  }
}
// do not allow two operators in a row
function checkOperator() {
  const lastChar = equation.charAt(equation.length - 1);
  for (let i = 0; i < operators.length; i++) {
    if (operators[i] === lastChar) {
      const remove = equation.substr(equation.length - 1, 1);
      equation = equation.replace(remove, operator);
      return;
    }
  }
  equation += operator;
  display.innerText = equation;
}
// clear calc
function clear() {
  display.innerHTML = "";
  solutions.innerHTML = "";
  numberString = "";
  equation = "";
  result = null;
}
!
999px

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