The gray border/line that separates the Treebar and Channel/Server/Query/whatever window can be edited via:

Right click in Treebar (Empty space, not a Channel/Server/Query/whatever icon/name) -> Background -> Select ...

You can use MS Paint or your favorite image editor and make a solid color image. Size does not matter it can be 10x10 pixels.

I do not know how to edit the scrollbar.