Someone asked me this recently, and further explained themselves:

I have a WordPress template that I’m using and I want to completely re-design a couple of pages (visual changes only). I can create the front-end layouts, but I’m interested in learning about how complex/lengthy the process can be to implement said front-end layout, or if it’s a fairly minimal task.

Here’s what I said back: It depends on how familiar with frameworks where the front-end is generated on the server-side (HAML with Ruby, say, as another example.) WordPress uses PHP based server-side rendering – so when you go look at the code for a theme, you’ll see HTML interspersed with PHP, on top of which you’ll see (PHP) calls to the WordPress API.

Also, there are best practices for how to introduce new Javascript or external CSS files (“assets.”) You can choose to not follow these guidelines but you might introduce subtle bugs.

If all of that is easy-peasy, then “re-designing” won’t be that hard, esp as you already know HTML/CSS.

It’s not that hard even if your experience is in “handle-bar style” frameworks like Angular. You still have to map things a bit into the hook/callback based setup you’ll see in WordPress but the learning curve is not that rough.

Otherwise, you’ll find that it’s a lot harder than say a “pure front-end” framework, where you probably only see HTML/CSS in some files, and then see only JavaScript in others.