CSS code to apply to MediaWiki:Wikia.css:

body {
    /* Multiple backgrounds stacked. Stacking is reversed (1st is top) */
    background-image: url(""), url("");
    background-repeat: no-repeat, repeat;
    background-attachment: fixed, scroll;
    background-position: center center, top left;
} {
    background-position: center bottom, left top;

To achieve the background style of this wiki. Upload File:Wiki-background and add it to Special:ThemeDesigner. Select tile but not fixed! In addition, upload File:Test Skin.png.

This has been created and modified for Bloons Wiki; can be used in a similar way for other wikis