To have the widest possible browser compatibility, my suggestion would be to create a transparent PNG image in an image editing program that has a white (or whatever) color with the level of opacity/transparency you want. I would suggest perhaps 10px x 10px, or even 5px x 5px. You then upload it to your media library, add the following to your custom CSS and replace IMAGE URL between the double quote marks with the URL of that image.
#page {
background: url("IMAGE URL") repeat scroll 0 0 transparent;
}
You might have to try a few different transparency/opacity levels to get a good balance that allows the image to show through, but keeps the text legible.