styles.css

*, *::before, *::after {
    box-sizing: border-box;
}

body {
    padding: 0;
    margin: 0;
    background-color: #708090;
    font-family: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
}

.calculator {
    display: grid;
    justify-content: center;
    align-content: center;
    min-height: 100vh;
    grid-template-columns: repeat(4, 100px);
    grid-template-rows: minmax(120px, auto) repeat(5, 100px);
}

.calculator-button {
    cursor: pointer;
    font-size: 2rem;    
    color: white;
    border: 0.1px solid black;
    outline: none;
    background-color: rgb(101, 107, 112);
}

.calculator-button:hover {
    background-color: rgba(101, 107, 112, 0.75);
}

.tools {
    background-color: rgb(61, 64, 70);
}

.tools:hover {
    background-color: rgba(61, 64, 70, 0.75);
}

.operators, .equals {
    background-color: #F1A43D;
}

.operators:hover, .equals:hover {
    background-color: rgba(255, 149, 0, 0.75);
}

.span-two {
    grid-column: span 2;
}

.output {
    grid-column: 1 / -1;
    background-color: rgba(50, 57, 64, 0.75);
    display: flex;
    align-items: flex-end;
    justify-content: space-around;
    flex-direction: column;
    padding: 10px;
    word-wrap: break-word;
    word-break: break-all;
}

.previous-num {
    color: rgba(255, 255, 255, .75);
    font-size: 1.5rem;
}

.current-num {
    color: white;
    font-size: 2.5rem;
}