style.css

body {
    background-color: aliceblue;
    box-sizing: border-box;
    padding: 0;
    margin: 0;

    /* main */
    display: flex;
    align-items: center;
    justify-content: center;
    height: 75vh;
    min-height: 500px;
    padding: 10px;
    margin-top: 50px;
}
#calculator {
    width: 400px;
    border: solid;
    border-radius: 15px;
    padding: 20px; 
    background-color: rgba(105, 109, 133, 0.84);
    box-shadow: 3px 3px 5px 3px rgba(0,0,0,0.5);
}

#screen {
    padding: 10px 20px;
    border: solid;
    border-radius: 5px;
    margin-bottom: 20px;
    background-color: rgb(149, 199, 149);
    font-size: 40px;
    min-width: 200px; 
    min-height: 100px;
}

#second-display {
    min-width: 50px; 
    min-height: 50px;
    font-size: 25px;
    text-align: right;
    word-wrap: break-word;
    word-break: break-all;
}

#display {
    text-align: right;
    word-wrap: break-word;
    word-break: break-all;
}

.all-buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    grid-gap: 10px;
}

#equals {
    grid-column: span 2;
}

button {
    padding: 20px;
    border: solid;
    border-radius: 40px;
    font-size: 25px;
    box-shadow: 2px 1px 4px 0px rgba(0,0,0,0.5);
}

button:hover {
    background-color: azure;
}

button:active {
    background-color: rgb(193, 203, 203);
    box-shadow: none;
    transform: translateY(4px);
}

#operators, #equals{
    background-color: rgb(67, 124, 185);
    border-color: black;
    color: white;
}
#operators:hover, #equals:hover {
    background-color: rgb(49, 87, 128);
}

#operators:active, #equals:active {
    background-color: rgb(82, 152, 226);
}