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.
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.