.particles {
    background: #fafafa;
    // background: url('../res/seigaiha.svg');
    background-size: 100px 50px;
    border-radius: 50px;
    height: 600px;
    margin: 10px auto;
    position: relative;
    width: 600px;
}

.particle {
    $side: 100px;

    background: url('../res/seahorse.svg') no-repeat center center;
    background-size: 20px 20px;
    border-color: salmon;
    border-style: dashed;
    border-radius: 50px;
    border-width: 1px;
    height: $side;
    position: absolute;
    width: $side;
}

.highlight {
    $h: 30px;
    animation: pulse 0.5s 1;
    border-radius: $h / 2;
    position: absolute;
}

@keyframes pulse {
    $h: 30px;

    from {
        border: 4px solid lightgreen;
        height: $h;
        margin: (-1 * $h / 2) 0 0 (-1 * $h / 2);
        width: $h;
    }

    to {
        height: 0px;
        margin: 0;
        width: 0px;
    }
}