But what I REALLY want is to put a vertical ad banner on the left / right of the forums. On a PC screen, there's a ton of unused space on either side of the forums.
I'm not very experienced in css etc, but I'm a fairly sharp fellow, and I figured I could work out how to put an ad banner there. I found the root css, and added a new section which created an ad banner. Sweet!
Unfortunately, this banner was anchored to the right-hand side of the screen, so if the window size were reduced, it would cover a bunch of the right-hand side of the forums. And it looked like a mess on mobile.
What I want is a block that sits to the right of the main forum area, and wouldn't overlap the forums even if the screen became too narrow to see this new block. I've got six (6) units of goodwill ready to deliver to any helpful person who can give me the secret to producing this layout properly!
EDIT: So, I see this question has been asked without answer before... fingers crossed that my luck is better! That first link has a really great image, in case there was any confusion about my goal.