<div class="toggleBtn"></div>
<div class="calculator">
  <div class="buttons">
    <h2 id="value"></h2>
    <span id="clear">Clear</span>
    <span>/</span>
    <span>*</span>
    <span>7</span>
    <span>8</span>
    <span>9</span>
    <span>-</span>
    <span>4</span>
    <span>5</span>
    <span>6</span>
    <span id="plus">+</span>
    <span>1</span>
    <span>2</span>
    <span>3</span>
    <span>0</span>
    <span>00</span>
    <span>.</span>
    <span id="equal">=</span>
  </div>
</div>

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

body  {
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  background: #edf1f4;
}

.calculator {
  position: relative;
  width: 340px;
  padding: 20px;
  border-radius: 20px;
  box-shadow: 15px 15px 20px rgba(0,0,0,0.1),
  -15px -15px 20px #fffb;
}

.calculator .buttons {
  position: relative;
  display: grid;
}

.calculator .buttons #value {
  position: relative;
  left: 8px;
  grid-column: span 4;
  height: 100px;
  line-height: 100px;
  padding: 0 20px;
  border-radius: 10px;
  text-align: end;
  font-size: 2em;
  margin-bottom: 12px;
  color: #5166d6;
  overflow: hidden;
  font-weight: 500;
  width: calc(100% - 16px);
  user-select: none;
  box-shadow: inset 5px 5px 10px rgba(0,0,0,0.1),
  inset -5px -5px 20px #fff;
}

.calculator .buttons span {
  position: relative;
  display: flex;
  justify-content: center;
  align-items: center;
  padding: 10px;
  border-radius: 10px;
  margin: 10px;
  min-width: 40px;
  font-size: 1.2em;
  color: #666;
  font-weight: 700;
  border: 2px solid #edf1f4;
  box-shadow: 5px 5px 10px rgba(0,0,0,0.1),
  -5px -5px 10px #fff;
  cursor: pointer;
  user-select: none;
}

.calculator .buttons span:active {
  color: #f44336;
  box-shadow: inset 5px 5px 10px rgba(0,0,0,0.1),
  inset -5px -5px 10px #fff;
}
.calculator .buttons span#clear {
  grid-column: span 2;
  background: #f44336;
  border: 2px solid #edf1f4;
  color: #fff;
}

.calculator .buttons span#plus {
  grid-row: span 2;
  background: #31a935;
  border: 2px solid #edf1f4;
  color: #fff;
}

.calculator .buttons span#equal {
  grid-row: span 2;
  background: #2196f3;
  border: 2px solid #edf1f4;
  color: #fff;
}

.calculator .buttons span#clear:active,
.calculator .buttons span#plus:active,
.calculator .buttons span#equal:active {
  box-shadow: 5px 5px 10px rgba(0,0,0,0.1),
  -5px -5px 10px #fff,
  inset 5px 5px 10px rgba(0,0,0,0.1);
}

.toggleBtn {
  position: fixed;
  top: 20px;
  right: 20px;
  background: #555;
  width: 20px;
  height: 20px;
  border-radius: 50%;
  cursor: pointer;
  border: 2px solid #edf1f4;
  box-shadow: 5px 5px 10px rgba(0,0,0,0.1),
  -5px -5px 10px #fff;
}

.dark .toggleBtn {
  background: #edf1f4;
  border: 2px solid #333;
  box-shadow: 5px 5px 10px rgba(0,0,0,0.5),
  -5px -5px 10px rgba(255,255,255,0.25);
}

.dark {
  background: #282c2f;
}

.dark .calculator {
  background: #33393e;
  box-shadow: 15px 15px 20px rgba(0,0,0,0.25),
  -15px -15px 20px rgba(255,255,255,0.1);
}

.dark .calculator #value {
  color: #eee;
  box-shadow: inset 5px 5px 10px rgba(0,0,0,0.5),
  inset -5px -5px 20px rgba(255,255,255,0.1);
}

.dark .calculator .buttons span {
  color: #eee;
  border: 2px solid #333;
  box-shadow: 5px 5px 10px rgba(0,0,0,0.25),
  -5px -5px 10px rgba(255,255,255,0.1);
}

.dark .calculator .buttons span:active {
  box-shadow: inset 5px 5px 10px rgba(0,0,0,0.25),
  inset -5px -5px 10px rgba(255,255,255,0.1);
}

.dark .calculator .buttons span#clear,
.dark .calculator .buttons span#plus,
.dark .calculator .buttons span#equal {
  border: 2px solid #333;;
}

.dark .calculator .buttons span#clear:active,
.dark .calculator .buttons span#plus:active,
.dark .calculator .buttons span#equal:active {
  box-shadow: inset 5px 5px 10px rgba(0,0,0,0.1);
}

let buttons =  document.querySelector(".buttons");
let btn = buttons.querySelectorAll("span");
let value = document.getElementById("value");
let toggleBtn = document.querySelector('.toggleBtn')
let body = document.querySelector('body')

for(let i=0; i<btn.length; i++){
  btn[i].addEventListener("click", function () { 

    if(this.innerHTML=="="){
      value.innerHTML = eval(value.innerHTML); 
    }else {
      if(this.innerHTML=='Clear'){
        value.innerHTML = "";
      } else{
        alert(this.innerHTML)
        value.innerHTML += this.innerHTML;
      }
    }
  });
}

toggleBtn.onclick = function(){
  body.classList.toggle('dark')
}