I'm designer by nature, I love to design. What you see as flat I see as simply beautiful :)
I know there are a LOT of things yet to fix and I'm waiting for some major changes in the Xenforo next release to implement them because I don't want to work double for my job to go to waste when I'll upgrade, either way, the forum software is already complex enough, if you don't paint complex with simplicity, the user can feel overwhelmed by everything and may want to go away.
The older way of doing things is not used anymore, things change. The design must fit a tablet, a smarphone, a big screen, if you make complex enough it will look bad on those devices and it will slow the site down in the process, finding a balance is really hard.
Anyway, I can assure you the design is going to get improved (a lot) on the course of this year, this is a very large project for me and I don't pretend to leave it abandoned at all. Besides, being the one in charge of design, servers and software licenses, I need to balance my time available.
I'm not going to ask people to dontate to improve the site, that's lame. Hell there are even other forums where the admins won't pay to make the forums.. what kind of commitment long-term can you expect from someone that does not want to pay for managing a good forum?.
I state this again, this project is very long-term, I'll take care of the payments, the software, the design. I only ask the community to enjoy it and give good pieces of advice in the Suggestion threads and from time to time, as the forum grows, get more people to join as moderators, report wrong post and ilegal stuff and we will turn into a real happy family.
The secret is always cooperation and helping each other. So, if you think the design is flat, try and be more specific and point where could improve in the suggestion thread as a new thread and I'll be happy to discuss what can be improved.