style.css

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

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

body {
  font-family: 'Montserrat', sans-serif;
  text-align: center;
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
  background-image: linear-gradient(315deg, #bdd4e7 0%, #8693ab 74%);
}

.calculator {
  background-color: #fff;
  width: 320px;
  border-radius: 20px;
}

input[type='text'] {
  height: 50px;
  width: 290px;
  margin-top: 20px;
  padding: 10px;
  border-radius: 10px;
  border: 1px solid grey;
  outline: none;
  text-align: right;
  font-weight: 500;
  font-size: 22px;
}

.buttons {
  width: 290px;
  margin: 20px auto;
}

input[type='button'] {
  width: calc(250px / 4);
  font-size: 20px;
  padding: 8px;
  margin: 3px;
  border-radius: 4px;
  border: 1px solid gray;
  cursor: pointer;
  transition: background-color 0.2s;
}

.clear input[type='button'] {
  width: calc(270px / 2);
}

input[type='button']:hover {
  background-color: lightgray;
}