There's no option, but you can instruct your browser to do it. Install the Stylish extension ([Chrome](https://chrome.google.com/webstore/detail/stylish/fjnbnpbmkenffdnngjfgmeleoegfcffe?hl=en), [Firefox](https://addons.mozilla.org/en-US/firefox/addon/stylish/)). In Firefox, you can use this code:

    @namespace url(http://www.w3.org/1999/xhtml);
    @-moz-document domain("askubuntu.com") {
        div#custom-header {
            display:none !important;
        }
    }

In Chrome, enter the following code, and specify that it applies to “URLs on the domain” `askubuntu.com`.

    div#custom-header {
        display:none !important;
    }

<sub>[Everything you ever wanted to know about custom CSS](http://superuser.com/questions/234952/understanding-css-for-user-styling-in-a-browser/465964#465964)</sub>