Question 1: What are your thoughts on the "theme" being used on the forum?
I like it; I really do. It's an almost "mystical" type theme, which fits your forum pretty well. The logo, header image, and favicon all look beautiful on my computer. However, your theme doesn't look very good at all when I change to my phone. The dark background combined with the yellow words doesn't work that well on lower pixelated devices. See
this screenshot as an example of what I'm talking about.
That same issue does kind of pop up, even on my PC. Especially on the navigation link, it's a little hard to read the text. Also - the link to the right of "Register" says "On", but leads to a login page. I'd guess that's some sort of mess-up in the code. It's not a huge deal, but I'd recommend you'd change it to something like "Login", regardless.
Question 2: What do you think of the way the forum is laid out?
Difficult question! When I first looked at the description above the navigation bar, I was honestly expecting a Harry Potter forum. However, I see that you only have one forum dedicated to Harry Potter. There's no "issue" with that, so to say, but it might be beneficial to add another Harry Potter section, even if it's just a subforum. A role playing section would be really cool, I think. If you can just get three or four people interested, you'd have a pretty solid section that people would login to see. That would also go with the name "Imagination Hub" pretty well.
Question 3: What would you do differently if you were running the forum?
That's a very broad question! I'd try my best to obtain a top level domain (TLD), if no other reason than to help with SEO. I'm not sure if you're interested in doing that; regardless, it would really help your forum. I'd also do my best to get content in your on topic sections. You have the most posts in the intro and off topic forums. I understand that it can be hard to convince people to post on topic, but doing something as simple as running a contest could go a long way if you're able to successfully promote it. I think that you're doing a great job, especially because your forum is still under a week old!