You can include the dark theme by importing it in the app.variables.scss file, like so:
// App Shared Variables
// --------------------------------------------------
// To customize the look and feel of this app, you can override
// the Sass variables found in Ionic's source scss files. Setting
// variables before Ionic's Sass will use these variables rather than
// Ionic's default Sass variable values. App Shared Sass imports belong
// in the app.core.scss file and not this file. Sass variables specific
// to the mode belong in the app.ios.scss, app.md.scss, or app.wp.scss files.
@import 'themes/dark';
or if you want to only import the theme for specific modes, you can import it in the mode specific app file. For example to use the dark theme only in Material Design, in app.md.scss:
// App Shared Variables
// --------------------------------------------------
// Shared Sass variables go in the app.variables.scss file
@import 'themes/dark.md';
@import 'app.variables';
The themes can be found here: https://github.com/driftyco/ionic/tree/2.0/src/themes