Nextpage in Posts

Hi,

When I use <!–nextpage–> (Page Break) in our posts, they look very small. I want them bigger, centered and with a background color. What is the easiest way for this without plugin? I am confused with functions.php, styles.css…

I need to change padding, background, text align, font size and font weight.

In which files, what are the codes that I should add or edit?

Thanks.