style.css

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  -webkit-font-smoothing: antialiased;
  -moz-font-smoothing: antialiased;
  -o-font-smoothing: antialiased;
  font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
  font-family: 'Roboto', sans-serif;
  font-size: 10px;
}

body {
  font-family: sans-serif;
  background-color: #fafafafa;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
}

.container {
  position: relative;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  width: 100%;
}

.calculatorl-container {
  background-color: rgb(44, 44, 44);
  padding: 5px 5px 5px 5px;
  border-radius: 30px;
  border: 15px solid orange;
  width: 350px;
}

.down-section {
  position: relative;
  height: 60vh;
  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
}

.output,
.input {
  position: relative;
  width: 98%;
  height: 60px;
  border: solid 1px black;
  margin: 5px;
  display: flex;
  align-items: center;
  padding: 5px 20px;
  font-size: 40px;
  background-color: cornflowerblue;
}

button {
  font-size: 22px;
  margin: 5px;
  outline-style: none;
  border: none;
  background-color: #fafafafa;
}
button:hover {
  filter: brightness(0.9);
}

.operator,
.equal,
.AC {
  background-color: orange;
}