I was wondered where you left all the possible themes?
Pluck 4.6 features a new, more robust and more feature-rich theming engine. This unfortunately causes old 4.5.x themes to not work with 4.6.
We've set up a new place where everyone can share their own themes, at the wiki: http://www.pluck-cms.org/docs/addons:start
There are no themes yet, but I hope that will change soon.