style.css

/* Reset and Box Sizing */
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

/* Body */
body {
  background-color: rgb(60, 57, 57);
}

/* Heading */
h1 {
  text-align: center;
  margin: 24px 0 18px 0;
  text-transform: uppercase;
  font-weight: 600;
  box-shadow: unset;
  font-family: "Lato", sans-serif !important;
  line-height: 21px;
  font-size: 32px;
  color: rgb(120, 226, 237);
}
h1 span {
  font-size: 15px;
  color: rgb(156, 215, 222);
}

/* Container */
.container {
  width: 460px;
  height: 540px;
  background-color: rgb(45, 44, 44);
  margin: auto;
  padding: 10px;
  border-radius: 10px;
}

/* Paragrapgh */
p {
  color: #fff;
  font-size: 1.6rem;
  font-family: "Franklin Gothic Medium", "Arial Narrow", Arial, sans-serif;
  margin: 10px 0 15px 10px;
  font-weight: 800;
}

/* Screen */
#screen {
  background-color: rgb(168, 213, 218);
  height: 80px;
  width: 440px;
  margin: 12px 0;
  font-size: 4rem;
  text-align: right;
  border-radius: 10px;
}

/* Button */
.button {
  background-color: rgb(62, 61, 61);
  color: white;
  width: 87px;
  height: 70px;
  font-size: 2.5rem;
  margin: 10px;
  border-radius: 15px;
  transition: all ease;
}
.button:hover {
  border: 3px solid rgb(164, 244, 44);
}

/* Additional Classes */
.star {
  text-align: center;
}
.clear {
  background-color: red;
}
.operator {
  background-color: grey;
}