[Brueskie]

Add a CSS selector called "fieldset" WITHOUT the "." at the beginning, and style it as you wish; do NOT edit the .fieldset class. For instance, add...

fieldset {
border: 1px solid #444444;
}

...in the Additional CSS Definitions field in the Style Manager.

Yes, in the editor_css template, you want to edit the .vBulletin_editor class and add the appropriate border to it. In my case, I used the same border property and value as in the above fieldset selector.