how to adjust the width of the forum div #content

[cannaculture]

I inspect the forums and see that this

#content {
    padding: 10px 20px 0;
    background: #FFFFFF none 0 0;
    width: 100%;
    min-width: 320px;
    max-width: 1200px;
    margin: auto;
    -moz-box-sizing: border-box;
    -webkit-box-sizing: border-box;
    box-sizing: border-box;
}

Controls the width. I change the 1200px to 1400px and the desired effect works. The forum stretches to 1400px as i need.

However how do i do this in the back admincp?

Any help is greatly appreciated!