Theme Catch Box transparant for background

Can you find an example of what you mean, a screenshot, a website etc. ?
Can you also link to your website or Pastebin.com both your HTML and CSS code?

You could apply transparency through CSS

selector {
	filter: alpha(opacity=50); /* internet explorer */
	-khtml-opacity: 0.5;      /* khtml, old safari */
	-moz-opacity: 0.5;       /* mozilla, netscape */
	opacity: 0.5;           /* fx, safari, opera */
	}

But there’s often a problem with the selector’s children elements inheriting the same transparency.
Here is a blog/tutorial about the children transparency
http://robertnyman.com/2010/01/11/css-background-transparency-without-affecting-child-elements-through-rgba-and-filters/

Or you could, through CSS, apply a 1px by 1px transparent image and have it repeat throughout the background.

To add CSS, you should be using the either;

☛ A Child Theme – http://codex.wordpress.org/Child_Themes
☛ A Custom Style plugin – http://wordpress.org/extend/plugins/custom-style/