You can find info on how to create/edit your theme at http://YOUR_DOMAIN/help/themes.html
If you been looking to download some theme from a website like you would do for WordPress then taught luck
From: Nimret Sandhu <nimret@...> To: Mailing list for Pebble weblog users <pebble-user@...> Sent: Tuesday, November 15, 2011 8:23 AM Subject: [pebble-user] themes?
now that I have it running under jetty, where can I find info on theme support in pebble? Ideally I am looking for some drop-in themes that I can hopefully choose from, rather than having to create my own.