style.css

.game {
    height: 500px;
    width: 70%;
    margin: 0 auto;
    border: 2px solid green;
    border-radius: 5px;
    overflow: hidden;
}

.bird {
    width: 40px;
    height: 40px;
    border-radius: 30px;
    position: absolute;
    top: 0;
    bottom: 0;
    margin: auto;
    margin-left: 10px;
    background-color: transparent;
}

.borders {
    float: right;
    height: 100%;
    width: 30px;
}

.border {
    width: 40px;
    margin-left: 35px;
    background-color: black;
}

.border-top {
    border-radius: 0px 4px 0px 0px;
}

.border-bottom {
    border-radius: 0px 0px 4px 0px;
}

.brend {
    padding-left: 200px;
    font-size: 45pt;
    margin-top: 0px;
    font-family: 'Slabo 27px', serif;
    letter-spacing: 10px;
}

.score {
    float: right;
    margin-right: 200px;
    font-size: 20pt;
}

.high-text {
    color: aqua;
}

.gameover-box {
    width: 300px;
    height: 300px;
    position: absolute;
    border: 1px solid black;
    border-radius: 5px;
    top: 0;
    left: 50%;
    margin-left: -170px;
    margin-top: 50px;
    display: none;
}

.gameover-box p,.restart {
    text-align: center;
    font-size: 20pt;
    font-family: 'Slabo 27px', serif;
}

.gameover-box .restart {
    cursor: pointer;
    background-color: aquamarine;
    position: absolute;
    bottom: 0;
    width: 100%;
    border-radius: 0px 0px 5px 5px;
}

.gameover-box .restart:hover {
    color: green;
}