My [non-gamer] wife appreciates that her PC is quiet, yet very fast for her needs. I'm not sure she really cares about the rather plain appearance of the Antec Sonata case, just that it works.
If you think she'd want to put decals on it, just build your wife's system in a plain case, possibly spray-painted (before assembly!) to whatever color she'd like. If she wants decals or some other covering on it, make sure she doesn't block the vents or fan openings.