application UI structure and style

application-wide UI structure

app/views/layouts/
application.html.erb
_header.html.erb
...

page-specific UI structure

app/views/users/
show.html.erb
index.html.erb
...

application-wide UI style

app/assets/stylesheets/
application.css
custom.css.scss
...

page-specific UI style

app/assets/stylesheets/
page.css.scss
...