#turn {
    color: grey;
}

#invalid-square {
    color: orange;
}

#title {
    text-align: center;
    font-family: helvetica, arial, sans-serif;
    background: linear-gradient(90deg, white 50%, black 50%);

}

canvas {
    display : block;
    margin : auto;
}

#box1 {
    padding: 30px;
    margin-left: 27px;
    color: green;
    font-weight: bold;
}

#box2 {
    padding: 30px;
    margin-left: 27px;
    color: green;
    font-weight: bold;
}

#start {
    width: 90px;
    height: 90px;
    border-radius: 45px;
    margin-left: 0px;
    background: linear-gradient(90deg, black 50%, white 50%);
    border-width: 10px;
    border-color: darkseagreen;
    color: cadetblue;
    font-size: medium;
    font-weight: bold;
}

#othello-right {
    color: white;
    width: 50%;
    float: right;
    text-align: left;
    font-size: 50px;
    margin-top: -40px;
}

#othello-left {
    color: black;
    width: 50%;
    float: left;
    text-align: right;
    font-size: 50px;
    margin-top: -40px;
}

#invalid-square, #turn {
    color: darkseagreen;
}

#title {
    text-align: center;
    font-family: helvetica, arial, sans-serif;
    background: linear-gradient(90deg, white 50%, black 50%);

}

canvas {
    display : block;
    margin : auto;
}

#box1 {
    padding: 30px;
    margin-left: 27px;
    color: green;
    font-weight: bold;
}

#box2 {
    padding: 30px;
    margin-left: 27px;
    color: green;
    font-weight: bold;
}

#start, #play-again {
    width: 90px;
    height: 90px;
    border-radius: 45px;
    margin-left: 0px;
    background: linear-gradient(90deg, black 50%, white 50%);
    border-width: 10px;
    border-color: darkseagreen;
    color: cadetblue;
    font-size: medium;
    font-weight: bold;
}

#othello-right {
    color: white;
    width: 50%;
    float: right;
    text-align: left;
    font-size: 50px;
    margin-top: -40px;
}

#othello-left {
    color: black;
    width: 50%;
    float: left;
    text-align: right;
    font-size: 50px;
    margin-top: -40px;
}

Make both messages sea green
Make both messages sea green

Both turn prompt and invalid square selection prompt.
