Option A — CDN / hosted link

Once you host techlapse-nova.css on Firebase, reference it directly:

<link rel="stylesheet"
  href="https://YOUR-PROJECT.web.app/techlapse-nova.css">

Option B — local file

Copy techlapse-nova.css into your project and link it:

<link rel="stylesheet" href="/css/techlapse-nova.css">

npm install @techlapse/nova
// then in your entry file:
import "@techlapse/nova/techlapse-nova.css";