style.css

body{
    font-family: 'Open Sans', sans-serif;
    background-color: black;
}

#calculator-container{
    width: 1000px;
    height: 600px;
    background-image: linear-gradient(rgba(0,0,0,0.3), rgba(0,0,0,0.3)), url(bg.jpg);
    margin: 0.5rem auto;
}

#calculator{
    width: 300px;
    height: 500px;
    background-color: azure;
    margin: 0 auto;
    position: relative;
    top: 15px;
    border-radius: 5px;
}

#result{
    height:120px;
}
#history{
    text-align: right;
    height: 20px;
    margin: 0 20px;
    padding-top: 20px;
    font-size: 15px;
    color: #919191;
    

}
#output{
    text-align: right;
    height: 60px;
    margin: 10px 20px;
    font-size: 30px;


}
#keypad{
    height: 380px;
}

.operator , .number, .empty{
    width: 45px;
    height: 45px; 
    margin: 15px;
    float: left;
    border-radius: 30%;
    border: none;
    font-weight: bold;
    font-size: 15px;
}

.number, .empty{
    background-color: inherit;
}

.number, .operator{
    cursor: pointer;
}
.number:active, .operator:active{
    font-size: 10px;
    
}

.number:focus, .operator:focus, .empty:focus{
    outline: none;
}

button:nth-child(4){
    font-size: 20px;
    background-color: #20b2aa;
}
button:nth-child(8){
    font-size: 20px;
    background-color: #ffa500;
}
button:nth-child(12){
    font-size: 20px;
    background-color: #f08080;
}
button:nth-child(16){
    font-size: 20px;
    background-color: #7d93e0;
}
button:nth-child(20){
    font-size: 20px;
    background-color: #9477af;
}