Flexibility 3 allows you to enter any custom CSS code directly in the theme options rather than having to manually edit your style sheet.

You can do just about anything using custom CSS code. Some of the customization to this site is done using custom CSS. For example, you can round corners using code like this:

.post {
-moz-border-radius: 8px; -webkit-border-radius:8px;
}
.posthead h2, .posthead h1 {
-moz-border-radius-topleft: 7px;
-moz-border-radius-topright: 7px;
-webkit-border-top-left-radius:7px;
-webkit-border-top-right-radius:7px;
}

This code will round the post box corners and also the top right and left corners of the post title. This effect is not visible in IE browsers, as IE does not support it.

You can apply corners to about any element on your page. I recommend downloading the Web Developer Toolbar plugin for FireFox so you can visually inspect page elements and find their ID and class values.

I recommend this site for learning how to work with CSS code.