Yup, HTML and CSS provide the skin for your app. HTML provides the structure of the UI (tags) and CSS provides the basic appearance (size, colors, position, etc.). When a web app is structured properly, you should be able to change/replace these things without affecting the brains (the actual script(s)) of the app whatsoever.
And once you arrive at that separation, you can do some cool stuff, like programmatically applying holiday themes based on the current date, or having your entire UI dynamically change based on the user’s browsing device (mobile vs. desktop), all without the underlying functionality of the site being affected at all.
And once you arrive at that separation, you can do some cool stuff, like programmatically applying holiday themes based on the current date, or having your entire UI dynamically change based on the user’s browsing device (mobile vs. desktop), all without the underlying functionality of the site being affected at all.