cyberpunk-theme.el: Use :background to set the cursor color

In a cyberpunk buffer (frame-parameter (selected-frame) 'cursor-color)
is nil because frame-parameter checks against the :background parameter,
not the :foreground parameter.  This is a problem in auto-complete-mode
where the frame cursor color may be saved and restored.
Merge pull request #6 from bzg/master

cyberpunk-theme.el: Use :background to set the cursor color