This is the most easiest way to develop quickly is by copy someone design and put in market to sell. They always have excuses with GPL. But I am strongly against this. I found here also few theme designers just copy popular theme design and tweak a bit add few new option and put in market. Using for personal use is different.
Same way copying stackoverflow design will may gives you some bucks but I believe if they have site design under CC license than they can go against your design. I believe which can be possible. After all they are very popular and your design also can grab attention of them.