in version 1.8.7-1 , config/qtcurveconfigbase.ui, line 2364
<string><p>If enabled, the titlebar buttons will be drawn using the cusom colors defined in the next page.</string>
cusom should be "custom".
This was originally reported in Ubuntu here:
Whoops, I don't know how you got here, but this tracker is completely unrelated to qtcurve.
The debian package info page for qtcurve is at http://packages.qa.debian.org/k/kde-style-qtcurve.html . Note that the package is marked as orphaned, there, so I guess it may be best to report the bug upstream. That appears to be at http://kde-look.org/content/show.php?content=40492 .
Regards
Thomas