I don't know much about Linux theming, but it seems there's an issue in the CSS for text colours. I'm using parrot-xfce. This is what it looks like using Icy-Dark (or any other parrot theme):

Using the Chicago95 theme:

Most other themes I've tried don't seem to have this problem. Some emails have white text (readable) interspersed with black text (unreadable), but some emails are all just black text, like this.
Thanks for the great work though! Love the theme!