A lot of users visit Wikia, and their screen resolutions all differ. What might look like a nice background to you, may look bad for a user with a larger resolution. Therefore, it's important to add gradients to your background before you upload it through the theme designer so the image will blend in with the background.
Gradients are smooth transitions from one colour to another. There are two types of gradients: linear ones, which is specified by two points, and a color at each point, and radial gradients, which is specified as a circle that has one color and a focus (the center of the circle) that has another. In this tutorial, we'll use linear gradients.
For this tutorial, we'll be using the image shown on the right, the well-known Windows background. You'll also need a program which supports gradients. In this case, we'll be using the free GIMP program.
Have a look at the right image, as you can see the blue background doesn't fit at all with the image background, and there's no transition at all between the two. To fix this, we'll first have to detect the background colour of the wiki. Take a screenshot of the page you're currently on (How to take a screenshot) and paste (CTRL+V) the image into GIMP.
In GIMP, select the "Colour Picker" from the toolbox on the left (). Now, click on the background colour of the wiki (not the background image). Now you've got the right colour to apply the gradient with.
Next, open the original background in GIMP (the windows desktop one). Choose the gradient tool (). Next, on the toolbox menu left, choose "FG to transparent" from the dropdown which appears by clicking on the image next to "Gradient:" (). Now you'll be able to apply the gradient.
Put your mouse on the left edge of the image, in the center (see image). Now hold the mouse button and pull to the very center of the image, and release the button. Now there's a gradient on the image. Do the same for the bottom and the right.
Now save the image, (File --> Save as...), and save the image as a .jpg file. In the dialogue menu that shows up, choose "Export". A new dialogue appears. There, check the "show example" checkbox. The filesize of the image will appear. Make sure it's below 100kB or you won't be able to save the background.
Go to Special:ThemeDesigner on your wiki and upload the image. Voilà, a nice looking background! Which blends in with the background!