Zen cart width and alignment?

OK, I read the following in the FAQs section:

How do I change the width of the template? (V1.3+)
FOR v1.3.x, assuming you've based your template on "template_default": open your stylesheet and find

#mainWrapper {
background-color: #ffffff;
text-align: left;
width: 100%;
vertical-align: top;
}

Simply change 100% to 750px or whatever width you desire

But changing that doesn't do anything to the width of my site. I am using a custom template based on template_default, so I only have one stylesheet. I did get the width to change by doing the following:

body {
margin: 0;
font-family: verdana, arial, helvetica, sans-serif;
font-size: 62.5%;
color: #000000;
background-color: #ffffff;
width: 75%;
}

But then the page is not centered, it is aligned to the left. What am I missing here?

Thanks!

P.S. This isn't for the site in my siggy :)

Try adding
text-align: center;

I had already tried that :) I fixed it finally by adding this:

body {
margin: 0;
font-family: verdana, arial, helvetica, sans-serif;
font-size: 62.5%;
color: #000000;
background-color: #ffffff;
width: 900px;
** margin: auto;**
}

I still don't know why it didn't work for me to change the #mainWrapper, but oh well, at least it worked somehow!

Thanks!