App: load CSS file from the assets folder

[YvanHK]

I'm creating an app (app-embed.liquid) where I want to have the CSS 'styles.css' in another folder:

From what I can read, we can achieve so for conditional app blocks but not sure it is working for my case:

I added the <link> above and the schema tag.

{% schema %}
  {
    "name": "",
    "target": "head",
    "stylesheet": "styles.css",
    "settings": []
  }
{% endschema %}

Do I need to have all CSS, assets, JS and liquid in one file or I can split?

Thanks for your help.