Comment 8 for bug 733578

Simon, don't bother fixing this in the theme. Thunderbird does the right thing here. It chooses the system menu color (have a look under System Colors in https://developer.mozilla.org/en/CSS/color_value) as defined in task-tree.css, which is in our case white.
So you can either switch the theme menu color to a dark color (which isn't a real option since we use dark menu backgrounds) or you can make Thunderbird to choose another color for displaying tasks.

I agree that shipping our own task-tree.css is an overkill. Maybe one can override those values with the correct entries in userchrome.css or usercontent.css. Otherwise it has to be fixed upstream.