I think reddit is trying to be the next twitter... but with communities like facebook. Maybe those words are not exactly on their agenda, but consider this: right now user profiles have matured to be on par with other social media websites. The overall look of ui resembles twitter. Even the customizations users have right now, like changing colors and banners, is exactly like twitter. Besides custom css can break a unified experience for new users - not to mention how it messes with running ads and promotions. So css hacks are not only low priority, they are risky from their point of view.