Html text-align: center; doesn't work

Question

I have a problem here, I want to center the text #top_page h1 but it doesn't work and I don't know why. can anyone suggest some ideas? I tried everything but text-align: center; and float: right or left doesn't work either.

I searched on google for help and looked through the same questions on stack overflow but none of them solves the problem so i'm asking you guys for help.

body {
        background: url(img/bg.jpg) no-repeat center center fixed; 
      -webkit-background-size: cover;
      -moz-background-size: cover;
      -o-background-size: cover;
      background-size: cover;
      display: inline-block;
      font-family: 'Cabin', sans-serif;

    }

    a {
        text-decoration: none;
        color: black;
        font-family: 'Cabin', sans-serif;
    }

    li {
        float: right;
        margin-top: 3px;
        margin-right: 50px;
        color: white;
        font-family: 'Cabin', sans-serif;
        font-size: 15px;
    }

    header {
        display: block;
        position: fixed;
        top: 0px;
        left: 0px;
        width: 100%;
        height: 50px;
        background: white;
        font-family: 'Cabin', sans-serif;
        z-index: 999999;
    }

    a:hover {
        color: crimson;
        font-family: 'Cabin', sans-serif;
    }

    span {
        width: 100%;
    }

    #top_page h1 {
        font-size: 80px;
        color: white;
        margin-top: 250px;
        display: block;
        text-align: center;
        font-family: 'Cabin', sans-serif;
    }

    #top_page h3 {
        font-size: 40px;
        margin-top: -40px;
        color: white;
        text-align: center;
        margin-left: 315px;
        font-family: 'Droid Sans', sans-serif;
    }

Answer 1

Your header has 100% width but the body does not. Just add width: 100% to body.