Okay, I didn't specify exactly where because it will depend upon your template set. With a default template set, edit the forumhome template, and add the style to the table tag after the line
If you start a new table for each catagory like we do, this will go in a different place which will only exist on your template set ^_^. Basicly add that to any table that you want to make partially transparent.
And I cannot stress enough how much this slows down rendering on the average computer.