If you want to mimic the theme on the aqwiki site, then yes, that is the CSS you want to copy. Just replace the 2 images with your own that have the same dimensions.
http://i41.tinypic.com/hv5mb6.jpg is 663 x 130 (the AQWiki logo text). That image is actually a PNG disguised as a JPG (JPG images can't have transparent backgrounds). Not sure why this was done, but the software looks inside the file and figures it out anyway.
http://i43.tinypic.com/311tl6a.jpg (the background image) is 1254 x 562, but only the top 162px or so is actually visible - your image probably doesn't need to be that tall.
The custom CSS used by AQWiki is extending the Shiny theme.
Is this enough to push you over the edge or do you need more step-by-step details to make this work? Remember, the best way to learn this stuff is to roll up your sleeves and dig in! :)
-Ed