[Elmer]

Yo can edit that border by editing the file vbulletin_editor.css located in the clientscript folder.

Look for

#vBulletin_editor
{
border: 2px groove;
}

and just change the border 'groove' by 'solid' or whatever you want.

note that you can not change the color to a groove border so instead of a groove border use solid borders like

border-bottom: solid 1px white;
border-right: solid 1px white;
border-top: solid 1px silver;
border-left: solid 1px silver;

to create an inset or outset border playing with it

or just

border: solid 2px yourcolor;