Board Thread:Support Requests - Getting Technical/@comment-24001421-20150905223447/@comment-17725354-20150906000455

Google Chrome recently did an update that reset your default zoom back to 100% (at least for me, I keep mine at 125%). I'm guessing that it changed yours as well—and thus your background suddenly looked different when in fact it never changed.

To solve this, you can either: If you do #1, make sure you turn off tiling so it shows a solid color outside the image. I know a thing or two about image editing so I can help you add the gradients if need be.
 * 1) edit the image so the sides are gradients that blend into your wiki's solid color background (example of this on my wiki here), or
 * 2) make the image bigger (not recommended; increases file size).