Hello the Wizard,
You are almost there! Here is what I do when I run into a problem like this: I start up whatever developers tools come with my browser. In IE and Chrome, the "F12" key starts up developers tools, which come with n "element selector" or "element inspector". These can tell you a lot.
For example, if I point to the page text on a simpleportal page, I immediately see that that content is inside a windowbg container, and that there are other parts inside there besides the content itself.
For example, at
http://simpleportal.net/index.php?page=about, I see the content is inside
<div class="windowbg">
<span class="topslice"><span></span></span>
<div class="content">
.... your content goes here ...
<>
<span class="botslice"><span></span></span>
<>
It sounds like you have added a custom class
windowbg4 for your page body.
And you have defined some custom CSS for windowbg4.
So now all you need to do is define some custom CSS for your topslice and botslice.
.windowbg4 span.topslice { }
.windowbg4 span.botslice {}
The CURVE theme uses curved images to make rounded corners. You can make an image file for that, or you can use border-radius instead (knowing that older browsers and Opera Mini will show square corners.
http://caniuse.com/border-radius).