Not at all. The idea is that the structure is HTML, the styling is CSS.
If you are old enough, think about webpages back in the 2000s. Or about how a page loads nowadays when your connection is poor: you basically bigger/smaller text, some underlined, blue (active) links and maybe a menu with a bullet list. That is the structure. The “menu” is in its core a list of contents, right? (as it is in a restaurant’s menu, or in the index of a school paper). The title is just text that you highlight (bigger, bolder) to let the reader know what a bunch of paragraphs is about. That is the structure of your page.
(If you’ve never seen a page like that, try CTRL+U (command+U) in any webpage, copy and paste the code to a local .html file in your computer and open it in a browser. You’ll see how all styling goes away and you’re left with pure structured content)
Now, if you take the exact same content and apply different stylesheets, you can have a website look completly different, though the content and its structure remain the same. Think of a website that was built in the 2000s, ugly AF, and then got refurbished a few years later. It now looks much prettier, but the content is still the same.
You if you wanna look further, this concept is called separation of concerns. That is: you got structure/content on one file (.html), style on another (.css), interactivity on another (.js) and so on and so forth
Hope that helped, if I was too confusing, let me know and I’ll try to be more clear =]