I tried the creation of a child theme, but nothing happens when I set the div tag parameters.
I think it's a CSS specificity problem and that your CSS is not spot on.
Luckily there is an easy way to test if your child theme CSS is working or not.
Create a child theme like I outline above, try using this in your child theme's style.css file:
/*
Theme Name: Showcase Child Theme
Description: Child theme for the Showcase theme
Author: Your name here
Template: showcase
*/
@import url("../showcase/style.css");
#content div.body {
background: #f00;
}
Activate that child theme for Showcase and visit your blog. Scroll down to the content and posts.
If you eye's begin to hurt and you wish you hadn't seen that, then your child theme is working correctly. :) I just tried it myself on my Crash Test Dummy and my eyes are still watering.
After that try using the Web Developer and Edit CSS plugins in Firefox to figure out the CSS for what you want to change. Once you've got the CSS, update your child theme style.css and you should be good to go.
https://addons.mozilla.org/en-US/firefox/addon/web-developer/
https://addons.mozilla.org/en-US/firefox/addon/editcss/