我试图在所有css类中使用边距底部来向下移动页眉和页脚中的内容，但仍然无法工作。请看图像来理解我的意思。CSS初学者在这里！我在下面包括html和css代码，并在css中评论了我试图插入边距的地方。谢谢。

   Please do help me see where I can insert margin-bottom so as my content in header and 
   footer can be shifted down a little 

 /* Add some padding on document's body to prevent the content
    to go underneath the header and footer */
    body{        
        padding-top: 60px;
        padding-bottom: 40px;
    }
/* I have tried to put margin-bottom here but it does not work */
    .container{
        width: 80%;
        margin: 0 auto; /* Center the DIV horizontally */
        text-align: center;
    }
/* I have tried to put margin-bottom here but it does not work */
    .fixed-header, .fixed-footer{
        width: ;
        height: 100px;
        position: fixed;        
        background: #333;
        padding: 10px 0;
        color: #fff;
        font-family:'Trebuchet MS', 'Lucida Sans Unicode', 'Lucida Grande', 'Lucida Sans', Arial, sans-serif;
        font-weight: bold;
    }
    .fixed-header{
        top: 0;
    }
    .fixed-footer{
        bottom: 0;
    }    
    /* Some more styles to beutify this example */
/* I have tried to put margin-bottom here but it does not work */
    nav a{
        color: #fff;
        text-decoration: none;
        padding: 10px 30px;
        display: inline-block;
    }
    .container p{
        line-height: 200px; /* Create scrollbar to test positioning */
        
    }