style.css

*,
*::before,
*::after {
  font-family: sans-serif;
  font-size: 10px;
}
body {
  background-image: url('img/blurry-gradient-haikei.svg');
  background-repeat: no-repeat;
  background-size: cover;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
}
h1 {
  display: flex;
  align-content: center;
  text-align: center;
  font-weight: 600;
  font-size: 2.5em;
  color: white;
}
.layout {
  margin: auto;
  width: 350px;
  height: 540px;
  position: relative;
  background: rgb(211, 215, 255);
  border-radius: 20px;
  justify-content: center;
  box-shadow: 15px 10px 25px rgba(138, 146, 231, 0.424);
}
.calculator {
  height: 500px;
  display: grid;
  justify-content: center;
  grid-template-columns: repeat(4, 70px);
  grid-template-rows: minmax(100px, auto) repeat(5, 70px);
}
.calculator > button {
  margin: 6px;
  color: #fff;
  font-size: 2rem;
  outline: none;
  cursor: pointer;
}
.span-two {
  grid-column: span 2;
  height: 90%;
  width: 95%;
  border-radius: 5em;
}
button {
  border-radius: 50%;
  border: none;
  background: linear-gradient(315deg, #7397fc, rgb(192, 207, 255));
  box-shadow: 2px 2px 6px #8caff0;
}
button:active {
  background: linear-gradient(315deg, rgb(185, 160, 243), #738cfc);
}
.output {
  grid-column: 1/ -1;
  background-color: #809eee3b;
  box-shadow: inset -2px 0px 5px #aabfe791;
  display: flex;
  align-items: flex-end;
  justify-content: space-around;
  flex-direction: column;
  padding: 0px 20px;
  word-wrap: break-word;
  word-break: break-all;
  border-radius: 10px;
  margin: 35px 0 30px 0;
}
.output .previous {
  color: rgb(255, 255, 255, 0.7);
  font-size: 20px;
  margin-top: 10px;
}
.output .current {
  color: #fff;
  font-size: 25px;
  margin: 5px 0;
}

@media screen and (max-width: 600px) {

  .layout {
    width: 250px;
    height: 400px;
    margin: 20px;
  }
  .calculator {
    height: 380px;
    display: grid;
    justify-content: center;
    grid-template-columns: repeat(4, 56px);
    grid-template-rows: minmax(35px, auto) repeat(5, 50px);
  }
  .calculator > button {
    font-size: 1.5rem;
  }
  .span-two {
    height: 75%;
    width: 90%;
  }
}