I'm going to assume you mean the default chat frame that you're trying to read. Mouse over the bulk of the text till the tabs pop up on top (one for chat and one for combat by default) right click on the chat one, and either in that popup, or the one under settings, there should be a color box to click on and set the background color. Don't mess with the color, but drag the slider next to the color wheel up to the top, that should increase the background's alpha.
__________________
Perhaps...
|