How to place a button in the center of div of dynamic size

I have this code:

<div class="outer">
    <ul class="list">
        <li class="inner">
            <div class="line">information1</div>
            <div class="line">hyperlink1</div>
        </li>
        <li>
            <div class="line">information2</div>
            <div class="line">hyperlink2</div>
        </li>
        <li>
            <div class="line">information3</div>
            <div class="line">hyperlink3</div>
        </li>
        <li>
            <div class="line">information4</div>
            <div class="line">hyperlink4</div>
        </li>
    </ul>
    <input type="submit" id="send" value="send" class="button"/>
</div>

and the css:

.outer
{
    display: table;
    border: 1px solid;
    padding: 5px;
}    
.list
{
    list-style: none;
    padding: 5px;
}    
.inner
{
    display: table-row;
    padding: 5px;
}    
.line
{
    display: table-cell;
    border: 1px solid;
    border-radius: 5px;
}    
.button
{
    top:50%; 
    left:50%;
}

the output is: this

Now i want to place the button in the center of the 'outer' div no matter what is the width.

example: i want the button to be in center even for: this. without having to change the css each time the div size changes. Even if the 'outer' div is dynamic, the button should be in the center.

thank you.