style.css

body{
    text-align: center;
    background-color: black;
    position: relative;
}

h1{
    font-family: Arial, Helvetica, sans-serif;
    color: blueviolet;
}

.square{
    width: 100px;
    height: 100px;
    background-color: blueviolet;
    border: 1px solid white;
    display: inline-block;
}

.o{
    position: relative;
}

.o:after{
    position: absolute;
    content: "\1F52E";
    font-size: 50px;
    line-height: 100px;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
}

.x{
    position: relative;
}

.x:after{
    position: absolute;
    content: "\2694";
    font-size: 70px;
    line-height: 100px;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
}

.reset{
    position: relative;
    left: 350px;
    bottom: 210px;
    height: 100px;
    line-height: 100px;
    width: 100px;
    border-radius: 50%;
    background-color: red;
    cursor: pointer;
    text-align: center;
    font-weight: bold;
}