Since Avalon is built on top of Twitter Bootstrap, customizing the interface to match your own theme is very straightforward. Within the file app/assets/stylesheets/branding.css.sass you can override any default Bootstrap variables. Since it will respect your defaults they wil be used when calculating the color schemes, grid sizes, and other places throughout the front end.
Avalon uses Twitter Bootstrap (http://twitter.github.com/bootstrap/), Hydra (http://projecthydra.org/), Blacklight (http://projectblacklight.org/) and custom Sass styles for its presentation. Stylesheets can be found in the app > assets > stylesheets directory.
Edit the branding.css.scss to customize the presentation or override default styles per your needs. Styles in branding.css.scc set variables that are used in other stylesheets and override default Bootstrap and Blacklight styles. All stylesheets are imported into application.scss, which prefers branding.css.scss over the others.