style.css

* { /*t'rn teeb seda, et see on k]ikidele elementidele*/
    font-family: Arial;
    font-weight: bold;
}

#calculator{
    width:350px;
    margin:100px auto;
    padding-top: 8px;
    padding-left:14px;
    background:linear-gradient(#9dd2ea,#8bceec);
    border-radius: 5px;

    /*disable user select*/
    -webkit-user-select: none;
    -moz-user-select: none;
    -ms-user-select: none;
    user-select: none;
}

#buttons span,
#top span {
    position: relative;
    top:0;

    display: inline-block;
    width: 70px;
    height: 40px;
    background: white;
    margin-right: 14px;
    margin-bottom: 8px;

    text-align: center;
    line-height: 40px;
    color: grey;
    border-radius: 5px;
    box-shadow: 0px 4px rgba(0,0,0,0.2);

    cursor: pointer;
    transition: all 0.2s ease;
}
#buttons span:hover,
#top span:hover {
    color: white;
    background: #9c89f6;
    box-shadow: 0px 4px #6b54d3;
}
#buttons span:active
#top span:active {
    box-shadow: none;
    top: 4px
}
#screen {
    float: right;
    background: rgba(0,0,0,0.2);
    width: 224px;
    height: 40px;
    margin-right: 18px;
    padding: 0 10px;
    box-shadow: inset 0px 4px rgba(0,0,0,0.2);

    border-radius: 5px;
    text-align: right;
    line-height: 40px;
    color: white;
    font-size: 20px;
    letter-spacing: 1px;
    text-shadow: 1px 1px 2px rgba(0,0,0,0.2);
}
#correct {
    width: 60px;
    height: 60px;
    text-align: center;
    line-height: 60px;
    margin: 0 auto;
    background: grey;
}