Version: 2.2 (using KDE Devel) Installed from: Compiled sources The button text on the Square Root button is not controlled by the theme, so it is black in every theme. (see e.g. Obsidian Coast)
I can confirm this bug.
SVN commit 770541 by brandybuck: BUG: 155275 Follow color scheme by using rich text for square root button M +0 -1 CMakeLists.txt M +2 -4 kcalc.cpp D pics (directory) WebSVN link: http://websvn.kde.org/?view=rev&revision=770541