You can give your page a background colour or image with ease using composer.
Witth both 6.x and 4.x versions click Format -> Page color and Properties. Click the colors and background tag.
You can select the background colour or image.
If you fix the colour you can get any colour you want (blue, white and so on). Make sure you pick wisely (shocking green doesn't do to well for the eyes to look at
Some people put backgound images on their pages. Usually they are not an image that covers the whole page but one that will tile together to give a consistent finish. By doing so they take up less space. (Remember if using an image, put it in your website folder before selecting it)
for example this image
Will tile out as below
By doing this they take up less space and are quicker to download. We use a varient of the image below(without the border) which is a mere 137 bytes
You can acquire background images from many sites (check our links).
With 4.x versions of composer you can also set a background image via the image properties menu
Tick the (A)" set as background box" on image properties