If you were to write it starting from a blank page you would need to add the scope yourself, the fact that the extension can add it for you does not mean it’s not needed. Also, i don’t really know the extension, i just looked at the documentation.
Look at this to see how it is done (or just install it if you are not doing it to learn but just want the functionality).
It is not incredibly hard, and what do you mean nobody knows? I already gave a link to how it is done, your task is to learn by looking at the CSS in the link i gave, and learn how to use the dev tools. I already asked you if you are asking to learn and you said yes. So go learn. If i give you all the steps i am giving you the answer, that is not learning, that is simply being shown how something is done.
You come to me and say, man i really want to learn to play this song on the guitar. I then proceed to play the song, look up at you and say “there now you know how to play the song”. No, you don’t know how to play the song, now do you?
You won’t learn anything with that kind of attitude. Your definition of hard seems to be something that you have to actually apply yourself to learn, if so yes this is hard, you won’t get it for free and it will take some effort.
There are tons of good resources, this is the Internet, you can basically learn anything from baking a cake to rocket science (rocket science is hard). Web development learning resources are no exception, on the contrary.
You do not know CSS so you will not be able to write your own styles.
The link to FreeCodeCamp in that statement was for people that might want to learn CSS. Since you don’t seem interested in learning CSS then there is really not much for you here.
That still leaves you with some choices though. You can find and use a style written by someone that does know CSS (and possibly alter it to your liking), you can Google “centering with CSS” and probably come up with an adequate solution, or you can reconsider you goals and dedicate some time to learning a minimum of CSS that will allow you create a Stylus stylesheet at some time in the near future.