Style problem with components not in the root module

I’m trying to add a home-made keyboard with ion-buttons.
The keyboard is a NG2 component, declared in a specific module (KeyboardModule), which gives the following html:

  <div class='fixed-content'>
        <button ion-button>a component inside AppModule</button> <!-- this one has the expected style -->
      <button ion-button>a component inside another module</button> <!-- this one is displayed with no style --> 

I tried to isolate the problem by creating a new button component in the AppModule (the default one): the style is correct.
It seems the reason why I lose style is the fact that my Keyboard component is embedded in another module.

Any idea how to get the ionic style in my new module ?

sources here

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