How to add a CSS file to the theme hook in Application

[ReyDev]

I've hooked a theme in my application, And I'm going to add a css file to this theme.
I've added the css file ( here : myapp > dev > css ), but it doesn't attach automatically to the theme like plugins.

How can I do it?

[bfarber]

\IPS\Output::i()->cssFiles = array_merge( \IPS\Output::i()->cssFiles, \IPS\Theme::i()->css( 'css-file-name.css', 'myapp', 'front' ) );