style.css

*
{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: consolas;
}
body
{
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background: #333;    
}

.calculator{
    position: relative;
    width: 330px;
}

.calculator .buttons
{
    position: relative;
    display: grid;
}

.calculator .buttons #value
{
    position: relative;
    left: 8px;
    margin-bottom: 12px;
    grid-column: span 4;
    height: 100px;
    line-height: 100px;
    padding: 0 20px;
    border-radius: 10px;
    background: wheat;
    text-align: right;
    font-size: 1.5em;
    color: #3a3c2e;
    overflow: hidden;

    box-shadow: inset 0 6px 1px 0 rgba(0, 0, 0, 0.35),
    0 5px 5px rgba(0,0,0,0.5),
    0 15px 25px rgba(0, 0, 0, 0.35);
    user-select: none;
    width: calc(100% - 16px);

}

.calculator .buttons span
{
    position: relative;
    padding: 10px;
    margin: 6px;
    min-width: 40px;
    font-size: 1.1em;
    display: flex;
    justify-content: center;
    align-items: center;
    color: #999;
    border: 2px solid #222;
    border-radius: 6px;
    background: linear-gradient(#555353,#363535);
    box-shadow: inset 0 6px 1px 0 rgba(0, 0, 0, 0.35),
    0 5px 5px rgba(0,0,0,0.5),
    0 15px 25px rgba(0, 0, 0, 0.35);
    cursor: pointer;
    user-select: none;

}

.calculator .buttons span::before
{
    content: '';
    position: absolute;
    inset: 5px 3px;
    border-top: 1px solid #ccc;
    filter: blur(2px);

}

.calculator .buttons span:active
{
    box-shadow: inset 0 6px 1px 0 rgba(0, 0, 0, 0.35),
    0 5px 5px rgba(0,0,0,0.5),
    0 15px 25px rgba(0, 0, 0, 0.35);
    color: #fff;
    text-shadow: 0 0 5px #219cf3,
    0 0 8px #219cf3;
}

.calculator .buttons span#clear
{
    grid-column: span 2;
    background: #ee1909;
    color: #fff;
}

.calculator .buttons span#plus
{
    grid-row: span 2;
    background: orange;
    color: #fff;
}

.calculator .buttons span#equal
{
    grid-row: span 2;
    background: rgb(25, 165, 25);
    color: #fff;
}