style.css

body{
    width: 33%;
  }
  
  #res{
    background-color: lightgray;
    border: solid;
    height: 48px;
    font-size: 30px;
    font-family: 'Courier New', Courier, monospace;
    font-weight: 900;
  }
  
  button{
    width: 50%;
    height: 50px;
    font-size: 18px;
    font-family: 'Courier New', Courier, monospace;
    font-weight: 600;
    margin: 0px;
    float: left;
  }
  
  #btn0, #btn1{
    /* background-color: black; */
    /* color: white; */
    border: 1px solid black;
  }
  
  #btnClr, #btnEql{
    background-color: darkgreen;
    color: white;
  }
  
  #btnSum, #btnSub, #btnMul, #btnDiv{
    background-color: black;
    color: red;
  }
  .container {
    width: 500px;
    height: 300px;
    background-color: aliceblue;
    margin-left: 440px;
    margin-top: 90px;
  }
  .left-container {
    width: 580px;
    height: 250px;
  }

  .rights-container {
    width: 200px;
    height: 250px;
  }
  #btns {
    display: flex;
  }
  .right-btns {
    width: 70px;
    height: 125px;
  }

  h1 {
    margin-top: 160px;
    margin-left: 550px;
    width: 500px;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
  }