In Firefox + Ubuntu Studio + dark screen theme (blackbird), the post editor has a dark background and a dark font color.

When you use a dark screen theme, you find a LOT of websites with broken CSS this way -- either the font color is specified dark, but the background color is left default (becomes dark from the system theme), or the background color is specified light and the font color takes the system default (light).

Should be an easy fix.
hjh