To change the border on the outsidce, for all tables, click on "Edit Board Template", and put this in your header:
(Not in the "CSS" field, as EC would over-ride that)
That'll make it white.
For the "welcome, username" bar, change the colour in EC for "Settings Bar BG Color"
For the border colour between forums, change the "Forumlist Border Color" box.
for a step-through of each option in EC..