How to add css styles in app.css for ember js addon

I am creating ember addons and there are few css styles which this addon requires to work properly!

How do i add them so that when a consuming application uses this addon the particular css styles are directly added to the app.css file of the consuming application?

Eg: addon: dropdown-addon requires a style

.active-menu {
    background-color: #4183C4;
}

consuming application: online-form

when online-form is using dropdown-addon i want .active-menu to be added to app.css automatically!