This CSS stylesheet is very simple:
It overrides the background image of the #logo_homepage block by a new one without the duck.

The code is not very readable because I had to use a data URI to include the image.

The image I used is: http://duckduckgo.com/assets/logo_words.v101.png

Here is the original code:

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document domain("duckduckgo.com") {   // apply only on duckduckgo.com
    #logo_homepage {
        background: url("/assets/logo_words.v101.png") no-repeat scroll center top transparent !important;  // set the background image
        height: 50px !important;            // new height of the block, smaller than the default one to avoid background-repeat
        margin-bottom: 40px !important;     // copied on the default style
        padding-bottom: 0 !important;       // no border
        width: 310px !important;            // new width of the block, the default one is to small
    }
}

Have fun !