html - 如何自动居中固定元素？

Question

我目前正在尝试将标题菜单固定在屏幕顶部，即使在滚动时也是如此。问题是，每当我将位置设置为固定时，它就会失去居中， margin 对其没有影响。我希望能够使元素居中，同时在调整页面大小时或以较小的分辨率查看时仍使其能够自动居中。

有问题的 HTML 和 CSS:

*{
    margin: 0;
    padding: 0;
}
h1{
    font: bold 20px Tahoma
}
h2{
    font: bold 20px Tahoma
}
header, section, footer, aside, nav, article {
    display: block;
}
html{
    height: 100%;
    background: rgb(143, 143, 143);
}
body{
    width: 100%;
    display: flex;
    justify-content: center;
}
#big_wrapper{
    margin: 0 0;
    display: flex;
    flex-direction: column;
    flex: 100%;

}
#top_header{
    position: fixed;
    text-align: center;
    height: 120px;
    background: rgb(45,45,45);
    /*border-bottom: 15px solid rgba(79, 79, 79, 0.4);*/
    border-radius: 8px;
    padding: 10px;
    /*box-shadow: rgb(30, 30, 30) 10px 10px 10px;*/
    font-family: Impact, sans-serif;
    font-size: 40px;
    color: white;
}
#centering_example{
    color: #a00d01;
    font-size: 45px;
    text-shadow: rgba(0,0,0,.5) -3px 3px 0;
}
#new_div{
    display: flex;
    flex-direction: row;
}
#main_section{
    position: relative;
    z-index: -1;
    top: 140px;
    max-width: 1200px;
    border-top-left-radius: 10px;
    border-top-right-radius: 10px;
    flex: 1;
    margin: auto;
    margin-top: 20px;
    padding: 20px;
    background: #3c3c3c;
    box-shadow: rgb(30, 30, 30) 10px 10px 10px;
}
#the_footer{
    position: relative;
    top: 140px;
    max-width: 1200px;
    margin: auto;
    text-align: center;
    padding: 20px;
    border-bottom-left-radius: 10px;
    border-bottom-right-radius: 10px;
    border-top: 1px solid black;
    background: #3c3c3c;
    box-shadow: rgb(30, 30, 30) 10px 10px 10px;
}
#SELECTED{
    display: inline-block;
    margin: 6px;
    margin-left: 5px;
    margin-right: 5px;
    border: 1px solid black;
    border-top: 1px solid #000000;
    border-top-color: rgba(0, 0, 0, 0.7);
    background: rgba(36,34,31, 0.6);
    box-shadow: rgba(0,0,0,1) 0 0 0;;
    padding: 9px 18px;
    border-radius: 8px;
    color: #f8f8f8;
    font-size: 16px;
    text-decoration: none;
    vertical-align: middle;
}
.media_button{
    display: inline-block;
    margin: 6px;
    margin-left: 5px;
    margin-right: 5px;
    border: 1px solid black;
    border-top: 1px solid #000000;
    background: rgba(69, 69, 69, 0.8);
    padding: 3px 8px;
    border-radius: 8px;
    box-shadow: rgba(0,0,0,1) 3px 3px 0;
    color: #787878;
    font-size: 13px;
    text-decoration: none;
    vertical-align: middle;
}
.media_button:hover{
    background: rgba(59,59,59, 0.6);
    box-shadow: rgba(0,0,0,1) 2px 2px 0;
    color: #f8f8f8;
}
.media_button:active {
    border-top-color: rgba(0, 0, 0, 0.7);
    background: rgba(36, 34, 31, 0.6);
    box-shadow: rgba(0, 0, 0, 1) 0 0 0;
}
.button{
    display: inline-block;
    margin: 6px;
    margin-left: 5px;
    margin-right: 5px;
    border: 1px solid black;
    border-top: 1px solid #000000;
    background: rgba(69, 69, 69, 0.8);
    padding: 9px 18px;
    border-radius: 8px;
    box-shadow: rgba(0,0,0,1) 3px 3px 0;
    color: #787878;
    font-size: 16px;
    text-decoration: none;
    vertical-align: middle;
}
.button:hover{
    background: rgba(59,59,59, 0.6);
    box-shadow: rgba(0,0,0,1) 2px 2px 0;
    color: #f8f8f8;
}
.button:active{
    border-top-color: rgba(0, 0, 0, 0.7);
    background: rgba(36,34,31, 0.6);
    box-shadow: rgba(0,0,0,1) 0 0 0;
}

<div id="big_wrapper">
    <header id="top_header">
        <p id="centering_example">centering help</p>
        <a id="SELECTED" class="button" href="index.html">Home</a>
        <a class="button">Games</a>
        <a class="button">About us</a>
        <a class="button">Blog</a>

    </header>

    <div id="new_div">

        <section id="main_section">
            <article>
                <p>This is text for the index page of the website.</p>
                <br>
            </article>

        </section>

    </div>

    <footer id="the_footer">
        <h6>Footer text</h6>
        <a class="media_button" href="https://www.twitter.com">Twitter</a>
        <a class="media_button" href="https://www.facebook.com">Facebook</a>
        <a class="media_button" href="http://steamcommunity.com">Steam Group</a>
    </footer>
</div>

Answer 1

尝试给它一个 left:50%; 和一个 margin-left，它是宽度的一半。

left: 50%;
margin-left: -200px;

更新的 fiddle :

http://jsfiddle.net/x3vh99vg/1/

不使用硬编码边距的更好方法(由 zgood 在下面的评论中建议)是将元素在 x 轴上平移 50%；

transform:translateX(-50%);

fiddle :

http://jsfiddle.net/x3vh99vg/2/