style.css

@import url('https://fonts.googleapis.com/css2?family=Poppins:wght@200;300;400;500;600;700&display=swap');

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

body{

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

nav{
  width: 100%;
  padding: 12px 0;
  background: #ecf0f3;
  box-shadow: -3px -3px 7px #ffffff,
             3px 3px 5px #ceced1,
             inset -3px -3px 7px #ffffff,
             inset 3px 3px 5px #ceced1;
}
nav .menu{
  max-width: 1270px;
  margin: auto;
  padding: 0 20px;
  display: flex;
  justify-content: space-between;
  align-items: center;
}
.menu .logo a{
  font-size: 28px;
  font-weight: 600;
  text-decoration: none;
  color: #31344b;
}
.menu ul{
  list-style: none;
  display: flex;
}
.menu ul a{
  margin:0 8px;
  text-decoration: none;
  font-size: 18px;
  color: #31344b;
  font-weight: 400;
  display: inline-flex;
  padding: 10px 12px;
 box-shadow: -3px -3px 7px #ffffff,
            3px 3px 5px #ceced1;
  position: relative;
  transition: all 0.3s ease;
}
.menu ul a:hover:before{
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  box-shadow: inset -3px -3px 7px #ffffff,
              inset 3px 3px 5px #ceced1;
}


.calculator{
  position: relative;
  display: grid;
  background: #ededed;
  box-shadow: 6px 6px 17px #c9c9c9,
  -6px -6px 17px #ffffff;
  margin: 5px;
  padding: 20px;
  border-radius: 10px;
}

.display{
  width: 260px;
  height: 100px;
  background: #ededed;
  box-shadow: inset 6px 6px 17px #c9c9c9,
              inset -6px -6px 17px #ffffff;
  border: none;
  outline: none;
  padding: 10px;
  text-align: right;
  font-size: 40px;
  border-radius: 10px;
  grid-column: span 4;
  font-weight: 900;
}

span{
  width: 60px;
  height: 60px;
  border-radius: 50px;
  box-shadow: 6px 6px 17px #c9c9c9,
  -6px -6px 17px #ffffff;
  font-size: 1.1rem;
  color: #000;
  font-weight: 900;
  background: #ededed;
  margin: 5px;
  display: grid;
  place-items: center;
}

span:active{
  box-shadow: inset 6px 6px 17px #c9c9c9,
              inset -6px -6px 17px #ffffff;
}

.clear{
  grid-column: span 2;
  width: 120px;
  background: #ff3077;
}

.plus{
  grid-row: span 2;
  height: 120px;
}

.equal{
  background: #03b1ff;
}
.center{
  position: absolute;
  top: 50%;
  left: 50%;
  width: 100%;
  transform: translate(-50%, -50%);
  text-align: center;
}
.upper{
  font-size: 40px;
  font-weight: 600;
  color: #31344b;
}
.lower{
  font-size: 40px;
  font-weight: 600;
  color: #31344b;
}
.menu ul a:hover{
  color: #3498db;
}
nav label.btn{
  color: #31344b;
  font-size: 18px;
  cursor: pointer;
  display: none;
}
nav label.cancel{
  position: absolute;
  top: 25px;
  right: 30px;
}
#check{
  display: none;
}