Could we either have a way to re-style the grow box that appears when you use the "resize:" CSS rule, or at least have a default style that fits with the aqua scroll bar that gets generated? If scroll bars are styled, could that style apply to the grow box by default too?
I sugest a psudo-element that lets you control the resizer. This would allow all sorts of style changes, even image replacement. This psudo-element could be used to chnage the cursor, see bug 9601.