We'll publish an article on this in near future. If you nonetheless already want to experiment with themes, here are the absolute basics:
Location: all standard themes can be found inside the bundle of the application. In order to access them, first open the context menu over the Picture Arena application bundle in the finder, then choose "Show contents". In the Finder window now opening, navigate to "Contents/Resources/Webexport Themes". You find all themes there.
Creating own copies: In order to create an own copy of a theme, copy (not move!) the theme to a different location on your computer and rename it, e.g. to "ModifiedTheme.patheme". Now, another two modifications are required so Picture Arena does recognize your themes as individual entities. Themes are bundles as well: again, you'll gain access by using the "Show contents" context menu entry on the theme. What needs to be adapted here is the file Contents/ThemeSpec.plist - a XML file in property list format. Here, you'll have to enter your own value for the entries "PAThemeName" and "PAThemeUUID". You can enter any value of choice for the "PAThemeUUID" as long as it doesn't start with 10. Please contact us, if you want to publish your theme, so we can give you an ID that's unique worldwide. Important: Never modify or save themes directly inside the Picture Arena bundle!
Installing own themes: In order to install your own theme, please perform a double click on the respective theme. To renew an installed theme, you'll first have to delete the old version from ~/Pictures/Picture Arena/Webexport Themes/, then install the new version via a double click. This procedure will be easified in future versions.