How to add a frame into the sidebox?

[kobra]

If I understand what you are asking correctly
This is created with an image and css

.sideBoxContent {
	margin: 0;
	padding: 0 0.3em 1em 0.3em;
	background-image:url(../images/sidebox_content_bg.gif);
	background-repeat:no-repeat;
	background-position:bottom;
	line-height: normal;
	}