I have a few files with the same problem. Try turning off "Display Styles" in the preferences. In my case, it seems that the CSS (style sheet) is defining the text color as black or something close to it, which makes for poor contrast on a black background.
When you turn off styles, it should fallback to the colors you defined in stanza's preferences. You may lose italics or some other formatting, but you will be able to read the text, at least. A better long-term solution is to contact the person you received the file from and ask them to remove the color references in their style sheet.
The Bandit