CSS center content inside div

To center a div, set it's width to some value and add margin: auto.

#partners .wrap {
    width: 655px;
    margin: auto;
}

EDIT, you want to center the div contents, not the div itself. You need to change display property of h2, ul and li to inline, and remove the float: left.

#partners li, ul, h2 {
    display: inline;
    float: none;
}

Then, they will be layed out like normal text elements, and aligned according to text-align property of their container, which is what you want.

There are many ways to center any element. I listed some

1. Set it's width to some value and add margin: 0 auto.

.partners {
    width: 80%;
    margin: 0 auto;
}

2. Split into 3 column layout

.partners {
    width: 80%;
    margin-left: 10%;
}

3. Use bootstrap layout

<div class="row">
    <div class="col-sm-4"></div>
    <div class="col-sm-4">Your Content / Image here</div>
</div>

You just need

.parent-div { text-align: center }

Try using flexbox. As an example, the following code shows the CSS for the container div inside which the contents needs to be centered aligned:

Depending on the axis of the flexbox, you will need to align or justify items, read more at MDN

.absolute-center {
    display: -ms-flexbox;
    display: -webkit-flex;
    display: flex;

    -ms-flex-align: center;
    -webkit-align-items: center;
    -webkit-box-align: center;

    align-items: center;
    justify-content: center;
}

You just do CSS changes for parent div

.parent-div { 
        text-align: center;
        display: block;
}