OK, I know I'm asking this question because I'm an absolute beginner when it comes to CSS but here goes.
Many of the pages on my website (older ones created with GoLive) Are just <p> <img> <p> <img> and on and on.
A page I am working on is all nice and centred and then I select a <p> and go "edit rule" and go to box and change it from auto to 800px. I do this so the text doesn't go all the way across the page which makes it hard to read. When I hit apply it changes the box but the whole page goes from being centered to being left justified. Everything on the page is up against the left border. What on earth is going on?
Review the CSS Box model.
You didn't mention which property you changed so it's impossible to comment. For better answers, show us your code.
There is no such CSS property as box-width or box width.
Perhaps you mean CSS box-sizing?
In any case, the URL you posted contains neither.
I am selecting a <p> then going to Properties>CSS>Edit rule the following dialog appears
Which I change to this:
CSS Definition for body, td, th is not the paragraph tag. It's the whole enchilada.
You have effectively reduced entire page width to 800px and removed centering. Don't do that.