To state it more directly:
No, it is not inherently dangerous for an attacker to see JS and CSS. After all, the attacker or any other client must be able to see these files in order for the application to work at all!
Actually, good security would mean designing your app so that an attacker who has complete (read-only) access to the HTML, CSS, JS, the server-side scripts, the web server's source code, and the system's configuration still could not pull off an attack. Attackers can, in general, get access to all these things. But in practice, you can take measures to hide the server-side configuration and source code, and it will slow down someone who is not sufficiently determined. You cannot hide the HTML/CSS/JS and still expect the web application to work.