style.css

:root {
  --background-color-first: #f9544c;
  --background-color-second: #090c31;
  --calculator-color: #15173c;
  --input-placeholder-color: #9490ac;
  --equal-color: #f9544c;
  --button-hover-color: #000d2e;
}

* {
  padding: 0;
  margin: 0;
  box-sizing: border-box;
  font-family: 'Roboto Mono', monospace;
  color: #fff;
}

body {
  height: 100vh;
  font-size: 16px;
  line-height: 1.9;
  background: linear-gradient(to bottom, var(--background-color-first) 50%, var(--background-color-second) 50%);
}

.calculator {
  width: 25rem;
  background-color: var(--calculator-color);
  padding: 3.125rem 1.875rem;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  border-radius: .5rem;
  box-shadow: 0 1.25rem 3.125rem rgba(0, 5, 24, .4);
}

.calculator__display {
  width: 100%;
}

.calculator__display-input {
  width: 100%;
  padding: .938rem .625rem;
  text-align: right;
  border: none;
  background-color: transparent;
  font-size: 2.188rem;
}

.calculator__display-input::placeholder {
  color: var(--input-placeholder-color);
}

.calculator__buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 1.25rem;
  margin-top: 2.5rem;
}

.calculator__buttons input[type="button"] {
  font-size: 1.25rem;
  padding: 1.063rem;
  border: none;
  background-color: transparent;
  border-radius: .313rem;
  cursor: pointer;
  transition: all .08s ease;
}

.calculator__buttons input[type="button"]:hover {
  box-shadow: 0 .5rem 1.563rem var(--button-hover-color);
}

.calculator__buttons input[type="button"]:active {
  transform: scale(.96);
}

.calculator__buttons #equal {
  grid-row: span 2;
  background-color: var(--equal-color);
}

.calculator__buttons input[type="button"][value="0"] {
  grid-column: span 2;
}

.footer {
  position: absolute;
  bottom: 1.25rem;
  width: 100%;
  text-align: center;
  font-size: .688rem;
  cursor: default;
}

.footer span {
  display: block;
}

.footer__link:hover {
  text-decoration: none;
}

@media (max-width: 26.25rem) {
  .calculator {
    width: 100vw;
    top: 0;
    left: 0;
    transform: none;
    border-radius: 0;
    padding: 1.875rem .625rem;
  }
}

@media (max-height: 45rem) {
  .footer {
    display: none;
  }
}