/* ボタン全体 */
#page_top {
    width: 50px;
    height: 50px;
    position: fixed;
    right: 10px;
    bottom: 10px;
    background: #e25407;
    border-radius: 50%;
    opacity: 0.6;
}

/* Font Awesome */
#page_top::before {
    font-family: 'Font Awesome 5 Free';
    font-weight: 900;
    content: '\f102';
    font-size: 20px;
    color: #ffffff;
    position: absolute;
    top: 50%;
    left: 50%;
    translate: -50% -50%;
}

.family1 {
    font-family: "Hiragino Kaku Gothic ProN", "Baskerville", sans-serif;
}

.family2 {
    font-family: 'Lucida Console';
}

#child_left_X {
    background-color: lightblue;
}

#child_right_X {
    background-color: lightgreen;
}

@media (min-width: 600px) {
    #parent_main {
        display: flex;
    }

    #child_left {
        flex-grow: 1;
    }

    #child_roght {
        flex-grow: 1;
    }
}