ember-bootstrap addon. What is this optional theme and where can I learn more about it?

I'm in the middle of installing the ember-bootstrap addon.

I'm following this guide: http://ift.tt/1Ti7Jow

Right now I'm assuming that bootstrap has installed correctly because my font and styling has changed on my page. However, I am curious about this part in the tutorial.

"Importing Twitter Bootstrap Theme

Bootstrap comes with an optional theme CSS with various visual enhancements. To include this file you can import it by setting importBootstrapTheme to true in your ember-cli-build.js:"

What is this optional theme and where can I read more about it? The docs are not very clear on what this optional theme is and I'd like to explore it a bit.