How to override styles of a library which has its own Activity

You can override the theme in your style file,

<style name="LibraryTheme" parent="Theme.AppCompat.Light.DarkActionBar">
    <item name="colorPrimary">Your color</item>
    <item name="colorPrimaryDark">Your color, too</item>
</style>

If "colorPrimary" will use the same value - @color/primaryColor, then value of library primaryColor will be overridden by primaryColor in sample app.

If you will use different values for "colorPrimary" in library and app, for example - @color/libPrimaryColor and @color/appPrimaryColor then colors will be different