I think we're about due for a cool custom design for this site. What needs to happen in order to make that a reality?
The short answer is "you gotta graduate from public beta before you can have a site design".
The long answer is the answer to a different question: "what does it take to graduate?". I wish I could tell you the exact and precise criteria for graduation, but there aren't any. In broad terms, it comes down to overall site growth over time and the quality of the questions and answers.
At the moment, Bitcoin.SE is generally on the right track. This year so far has brought you guys a healthy amount of new traffic. We'd like to see if it is sustainable over a longer term. Your userbase is small, but engaged. This is good, but there's definitely room to grow there.
A custom design aside, you guys right now are a fully-featured and functional Stack Exchange site. The term "beta" is more a historical artefact than anything.
You're asking and answering questions about Bitcoin and filling a niche in a (still) small but growing market. Keep doing what you're doing and maybe throw a bit of site promotion into the mix, and I have no doubt that you can get to the point where you can have your cool looks and eat them too. (Okay, this didn't come out quite right, but you get the idea.)