Hi @CKB_KY,
I agree with James that your message is a bit vague. You can have a custom CSS and a custom JavaScript file in your account theme, which is great, but it doesn't mean you can do anything and everything and doesn't necessarily mean it will be easy.
I got started dabbling in this area years ago as my institution wanted to hide some options form faculty back when permissions were not as granular as they are now. This is relatively easy to do with JavaScript and/or CSS, depending on the item you're trying to hide. Similarly, adding a little text somewhere can be relatively easy. I have a computer science degree, so while I hadn't dabbled in CSS or JavaScript in years, I still had an idea of how they worked in my head, knew what probably would be doable, and could "google search" my way through it...
When you're brand new to all of this, getting started can be hard. I'd recommend coming up with something you'd like to tweak about Canvas. I say tweak because without a lot of experience, making major changes/additions/removals is probably going to be out of the question, and CSS/JavaScript are still very limited in what they can accomplish even for skilled developers. If you have a tweak in mind, you could let us know what that tweak is here, and someone may reply with some pointers. I'm sure there are some CSS/JavaScript courses available somewhere, but I'm generally a learn by doing type of person, so the courses aren't very helpful to me.
Once you accomplish one or two tweaks, you might start getting a better feel for things, be able to determine what's doable, what's probably not, etc... I'd encourage you to do as much as you can on your own, but if you get stuck, the Developers Gorup here in the Instructure Community is a great place to ask for help. No guarantees that someone will reply to your question, but there are some great people there.
Finally, I want to stress that as a Canvas admin myself, I know how CSS/JavaScript can go completely sideways as Canvas is constantly evolving. You may have code that works for years, then one day is totally broken because of a UI change. I think most admins would tell you to keep your CSS/JavaScript changes as minimal as possible to help avoid issues.
If you want some examples of small tweaks I've done (though a few may no longer be relevant), you can check my github repository.
Hope this info helps!
-Chris