CSS Center a Text Box [duplicate]

Question

Here is the registration box:

http://technicaldebt.co.uk/fyp/register.php

I am trying to get the box to center in the middle of the webpage. The CSS is attached below. Any help would be greatly appreciated.

/*********************************************************************************/
/* Basic                                                                         */
/*********************************************************************************/

*
{
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}

body
{
    background: #1e1e1e  url(images/bg04.jpg) repeat;
    font-family: 'Open Sans', sans-serif;
    font-size: 11pt;
    color: #7f7f7f;
}

form
{
}

    form input,
    form select,
    form textarea
    {
        -webkit-appearance: none;
    }

br.clear
{
    clear: both;
}

p, ul, ol, dl, table
{
    margin-bottom: 1em;
}

p
{
    line-height: 1.7em;
}

a
{
    color: #779c5b;
}

    a:hover
    {
        text-decoration: none;
    }

section,
article
{
    margin-bottom: 3em;
}

section > :last-child,
article > :last-child
{
    margin-bottom: 0;
}

section:last-child,
article:last-child
{
    margin-bottom: 0;
}

.image
{
    display: inline-block;
}

    .image img
    {
        display: block;
        width: 100%;
    }

.image-full
{
    display: block;
    width: 100%;
}

.image-left
{
    float: left;
    margin: 0 2em 0 1em;
}

.image-border img
{
    border-radius: 5px;
}

ul.style1
{
}

    ul.style1 li
    {
        padding: 0.80em 0 0.75em 0;
        border-top: 1px solid #e0e0e0;
    }

    ul.style1 a
    {
        text-decoration: underline;
        color: #779c5b;
    }

        ul.style1 a:hover
        {
            text-decoration: none;
        }

    ul.style1 .image-left
    {
        margin-top: 0.50em;
        margin-right: 1.50em;
        margin-left: 0 !important;
    }

    ul.style1 .date
    {
        display: block;
        margin: 0;
        padding: 1em 0 0 0;
        line-height: 0;
        color: #047ab7;
    }

    ul.style1 .first
    {
        border-top: none;
    }

ul.style2
{
}

    ul.style2 li
    {
        overflow: hidden;
        padding: 1.75em 0 1.75em 0;
        border-top: 1px solid #e0e0e0;
    }

    ul.style2 p
    {
        margin: 0;
    }

    ul.style2 h3 {
        padding: 0 0 0.50em 0;
        font-size: 1.00em;
    }

    ul.style2 .image-left
    {
        margin-left: 0;
    }

    ul.style2 .first
    {
        border-top: none;
    }

ol.style1
{
    margin-left: 3em;
}

    ol.style1 li
    {
        padding: 0.35em 0;
        list-style: decimal;
    }

.button
{
    display: inline-block;
    margin-top: 1em;
    padding: 0.70em 1.5em;
    border-radius: 5px;
    background: #779c5b;
    line-height: 1;
    text-align: center;
    text-decoration: none;
    color: #FFFFFF;
    transition: background-color .25s ease-in-out;
    -moz-transition: background-color .25s ease-in-out;
    -webkit-transition: background-color .25s ease-in-out;
}

    .button:hover
    {
        background: #96b77c;
    }

.button-style1
{
    background: #222222;
}

    .button-style1:hover
    {
        background: #2d2d2d;
    }

.button-style2
{
    margin-top: 0.50em;
    padding: 1.75em 3.00em;
    box-shadow: 5px 0 5px -5px rgba(0, 0, 0, 0.4), 0 6px 5px -5px rgba(0, 0, 0, 0.4), -5px 0 5px -5px rgba(0, 0, 0, 0.4);
    line-height: 0.25em;
}
    .button-style2:hover
    {
        background: #96b77c;
    }

/* Assign these to ARTICLE tags */

.box
{
    padding: 2.50em 2.50em;
    background: #FFFFFF;
    border: 1px solid #e0e0e0;
    border-radius: 5px;
}

    .box h2
    {
        padding: 0 0 0.40em 0;
        letter-spacing: -0.03em;
        font-size: 1.60em;
        color: #0f0f0f;
    }

    .box .subtitle
    {
        padding: 0 0 0.30em 0;
        font-size: 1.10em;
        color: #5f6b8b;
    }

.box-post
{
}

.box-featured-post
{
}

/* Assign these to SECTION tags */

.box-news
{
}

.box-tweets
{
}

.box-contact
{
}

/*********************************************************************************/
/* Wrappers                                                                      */
/*********************************************************************************/

#wrapper
{
    overflow: hidden;
    background: url(images/bg01.jpg) repeat;
}

#wrapper-gradient
{
}

#header-wrapper
{
    overflow: hidden;

    background: url(images/gradient.svg);

    background: -moz-linear-gradient(
            center bottom,
            rgba(0,0,0,0) 5%,
            rgba(0,0,0,0.5) 100%
        );

    background: -webkit-gradient(
        linear,
        left bottom,
        left top,
        color-stop(0.0, rgba(0,0,0,0.0)),
        color-stop(1.0, rgba(0,0,0,0.5))
    );
}

#banner-wrapper
{
    overflow: hidden;
}

#feature-wrapper
{
    overflow: hidden;
    padding: 3em 0em;
    background: #5f6b8b url(images/shadow02.png) no-repeat center top;
    box-shadow: 0px 10px 5px rgba(0,0,0,.3), 0px -1px 25px rgba(0,0,0,.3);
    color: #dfe2e8;
}

#main-wrapper
{
    overflow: hidden;
    padding: 3em 0em 4em 0em;
    background: #f3f3f3 url(images/bg03.jpg) repeat;
    box-shadow: 0px 10px 5px rgba(0,0,0,.3), 0px -1px 25px rgba(0,0,0,.3);
}

#footer-wrapper
{
}

#copyright-wrapper
{

    background: url(images/gradient.svg);

    background:
        -moz-linear-gradient(
            center bottom,
            rgba(0,0,0,0) 5%,
            rgba(0,0,0,0.5) 70%
        );

    background: -webkit-gradient(
        linear,
        left bottom,
        left top,
        color-stop(0.0, rgba(0,0,0,0.0)),
        color-stop(0.70, rgba(0,0,0,0.5))
    );      

}


/*********************************************************************************/
/* Feature                                                                       */
/*********************************************************************************/

#feature-content
{
}

#feature-content h2
{
    height: 3.50em;
    margin: 0 0 1em 0;
    padding: 0 0 0 3.50em;
    background: url(images/arrow01.png) no-repeat 0.75em 50%;
    border-bottom: 1px solid #949db3;
    letter-spacing: -0.02em;
    font-size: 1.50em;
    color: #FFFFFF;
}

#feature-content span
{
    display: inline-block;
    height: 3.50em;
    padding: 0 0 0 1em;
    border-left: 1px solid #949db3;
    line-height: 3.5em;
}


/*********************************************************************************/
/* Banner                                                                        */
/*********************************************************************************/

#banner
{
    overflow: hidden;
}

#banner h2
{
    display: inline-block;
    font-weight: 700;
    color: #FFFFFF;
}

#banner .subtitle
{
    font-weight: 300;
    color: #ababab;
}

/*********************************************************************************/
/* Content                                                                       */
/*********************************************************************************/

#content
{
}

    #content article
    {
    }

    #content .image-left
    {
        margin-left: 0;
    }

/*********************************************************************************/
/* Sidebar                                                                       */
/*********************************************************************************/

#sidebar
{
}


/*********************************************************************************/
/* Two Column                                                                    */
/*********************************************************************************/


#two-column
{
}

    #two-column .tbox
    {
    }

        #two-column .tbox .image-full
        {
            padding-bottom: 2em;
        }


/*
Registration/Login Form by html-form-guide.com
You can customize all the aspects of the form in this style sheet
*/

#fg_membersite fieldset
{
width: 230px;
padding:20px;
border:1px solid #ccc;
-moz-border-radius: 10px;
-webkit-border-radius: 10px;
-khtml-border-radius: 10px;
border-radius: 10px;   
}

Answer 1

To center a div with margin: 0 auto; like suggested the div must contain a valid width.

If you add

#register{
 //...
 width: 300px;
 margin: 0 auto;
}

This will center your div in the center horizontally. You can also use text-align: center in the parent with display: inline-block in the form div. Such as:

#fg_membersite{
 text-align:center;
}

#register{
 display:inline-block;
}

If you want to center it also in the vertical i suggest you use display:table (on the aprent div) and display:table-cell (in a wrapper div) and vertical-align:middle options , or maybe a position:absolute with negative margins, it's your choice.

Cheers.