Comment 6 for bug 733578

As pointed out under http://getsatisfaction.com/mozilla_messaging/topics/in_lightning_how_to_change_the_text_color_for_task_lists respectively this problem occurs, because they choose the MenuText color for tasks in calendar-tasks-tree.css, which is for dark menu themes white (or a light color).

To fix this, one should choose e.g. WindowText as color in the sections:

.calendar-task-tree > treechildren::-moz-tree-cell-text(completed)
.calendar-task-tree > treechildren::-moz-tree-cell-text(future)

of calendar-tasks-tree.css.