I have spent quite a bit of time playing around and have not been able to get this to work.  I am such a noob with css...

I am sure this is really simple to do with css.

What would be the proper overriding css that I should use?

From what I can tell the element is:

<div class="logo_top">

Thanks in advance

it should be as simple as adding the following to your css (portal.css) - this will center the content within the div

.logo_top
{
align: center;
}

if you want to centre the div within the page, you have to supply a page width and then get the div to handle the margins automatically i.e.

.logo_top {
  width: 700px ;
  margin-left: auto ;
  margin-right: auto ;
}

if you're not doing a fixed width, you can use percentages

 .logo_top {
  width: 100% ;
  margin-left: auto ;
  margin-right: auto ;
}

Do NOT change the default skin, but make a copy of it and change that skin or else your skin changes will be overwritten if you update DNN