Hey,
I've created some images that will help you the wiki's wordmark and that will make the background expand to the whole page.
Wordmark
I made your current wordmark white, which is now visible when you put it on your dark header.
You can download the transparent image by right-clicking and clicking "save image".
Background
Being that the background was simply too small for some supported screens, it wouldn't fill the entire screen. Luckily, I was able to expand the wanted image, but I did have to blur it to make sure it didn't look pixilated but still had a clean look.
Click here to get the full-sized image. You can download the image by right-clicking and clicking "save image".
Good luck!