I would actually recommend a hybrid approach – take a good theme for showcasing photos and substitute in your YouTube videos.
Photo themes give you large portions of screen real estate for imagery, and if you do it right it’s not difficult to substitute an embedded object (i.e. YouTube video) for the images.
Some good ones to start with:
Alternatively, you can also find some pretty good video-specific themes that will take less work to rebuild and redesign:
- WooTube – a YouTube clone by WooThemes
- WordTube – another YouTube clone
- Video themes by ThemeGarden