I'm asking this because the graphic design site has been in public beta for 838 days and is still using the default beta-site theme which seems quite inappropriate for a site full of graphical designers.
Beta sites don't get custom looks until sometime after they exit the beta period. As beta sites, they get a default beta look, which is plain as you have noted. See the Graphic Design FAQ for more info.
In the long run (i.e. after the beta period), yes, the plain look would be quite ironic for a graphic-design site. :)