To create a Parley theme you need to edit a SVG file.
SVG files can contain multiple elements and they have to follow a naming convention so that parley can show the right items.
The easiest is to find a finished svg file and change it to your liking.
One theme you can get directly from kde's source code repository is the default one: http://websvn.kde.org/*checkout*/trunk/KDE/kdeedu/parley/themes/theme_reference.svgz This file contains some instructions how the items have to be named.
Other themes are here: http://kde-look.org/?xcontentmode=85
To test a theme you need three files: - themename.desktop which describes the theme with its name and points to the image files. - themename.png or .jpg as preview - themename.svgz is a compressed svg file which contains the actual graphics
The files have to be found by Parley, so they should be in a hidden directory at this location: /home/your_user_name/.kde/share/apps/parley/themes
When you created the three files you can start Parley and it should automatically find your new theme.