What you see as the home page is separated into several *.html.erb files:
app/views/catalog/_home_text.html.erb
app/views/modules/_header.html.erb
app/views/modules/_footer.html.erb
and they're all put together in the avalon layout: app/views/layouts/avalon.html.erb
Avalon uses project blacklight for its layout, so if you need to look up how to do something specific to the look, you can google for blacklight.
Changing the featured items
An *.html.erb file is just an html file with some embedded ruby.
This documentation is still in progress.