Can you post a link to the site or post some screenshots of your problem so we can get a better idea of what is going on?

The theme I used was “OceanWP”

According to the preview, everything looked right, but when I published it. The entire theme looked different. Like I said before, it was as if my CSS code affected all the other themes. The CSS code I used was…..something of like a entire style-sheet.

I think my problem was I didn’t inserted the CSS script in the right area.

It’s on my local host and I haven’t hosted the site anywhere yet. I couldn’t fix the problem so what I did was I just rebuilt the entire website from scratch.

By “Every other theme”, it just means what it means.
The CSS code I uploaded into the “Content” file affected every theme on wordpress.

*when I made this post, I was unaware of how changing themes in WordPress work*

As I was researching, I realized……these kinds of problem concerning dealing with “Themes” must be a norm.

However, the CSS you put in the customizer should stick with the theme current when it’s added.