style.css

body{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

.container{
    display: flex;
    justify-content: center;
    align-items: center;
    min-width: 100%;
    min-height: 100vh;
    background-color: black;
}
.game-container{
    width: 500px;
    height: 750px;
    position: relative;
    overflow: hidden;
}

.game-container .sky{
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 80%;
    background-image: url(Images/sky.png);
}

.game-container .land{
    position: absolute;
    bottom: 0;
    left: 0;
    width: 100%;
    height: 20%;
    background-image: url(Images/land.png);
    z-index: 100;
}

.bird{
    position: absolute;
    bottom: 250px;
    left: 200px;
    background-image: url(Images/bird.png);
    background-position: center;
    background-size: contain;
    background-repeat: no-repeat;
    width: 60px;
    height: 60px;
}

.obstacle{
    background-image: url(Images/pipe.png);
    position: absolute;
    width: 60px;
    height: 300px;
}

.top-obstacle{
    background-image: url(Images/pipe.png);
    transform: rotate(180deg);
    position: absolute;
    width: 60px;
    height: 300px;
}

.bird.up{
    transform: rotate(300deg);
}

.bird.down{
    transform: rotate(20deg);
}