style.css

html { 
    background: url(background.jpg) no-repeat center center fixed; 
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
  }

button  {
    font-family: 'Orbitron', sans-serif;
    height: 2em;
    font-size: 2em;
    box-shadow: 2px 4px #888888;
}

#container  {
    display: grid;
    justify-content: center;   
    vertical-align: middle; 
    margin:20%;
    width: 60%;
    /* border: 3px solid #73AD21; */
    
    
}
#calculator {
    background-color:lightgray;
    opacity: 90%;
    display: grid;
    width: 300px;
    height: 420px;  
    justify-content: center;  
    border: 2px solid gray;
    border-right: 2px solid black;
    border-bottom: 2px solid black;
    border-radius: 2%;
    padding: 10px 0px ;
    font-family: 'Orbitron', sans-serif;
    box-shadow: 5px 7px #888888;

}


#display   {
    display: flex;
    height: 1.2em;
    width: fit-content;
    border: 2px solid black;
    justify-content: right;
    margin-top: 10px;
    padding: 0px 10px;
    background-color: gray;
    color: #d7ff7e;
    font-size: 3em;
    box-shadow: 3px 4px darkgrey;
}

#topline {
    display: flex;
    justify-content: space-around;    
    
}

#secondline {
    display: flex;
    justify-content: space-around;      
}

#thirdline  {
    display: flex;
    justify-content: space-around;  
}

#fourthline {
    display: flex;
    justify-content: space-around;
    
}

.button {
    padding: .5em;
}