The flowplayer playlist is pure CSS styled HTML (with JS of course). The css file responsible is within `zp-core/zp-extensions/flowplayer3_playlist`. But you don't need to change that ifself, just copy the file to your theme's folder and change as needed. It will then be used instead of the original plugin one.
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Btw, this is all written on the plugin doc:
http://www.zenphoto.org/documentation/plugins/_zp-extensions---flowplayer3_playlist.php.html
For infos how playlist itself work please refer to the documentation on flowplayer.org.