nv.wiki is given as an example of a negative customization. This is a bit sad, because it is actually one of the nicest examples of customization. Not just because it is beautiful - that's a matter of taste - but because it shows a real need that some communities have: To customize their wiki to show something of their culture and to make it more comfortable, but without making a whole new skin. Because you know what happens when a new skin is made, right? - The developers, quite rightly, don't want to support it, and bam: VisualEditor doesn't work.
Arabic had a comparable cultural customization in the past, but the introduction of Vector broke it.
A clever separation of styles that takes into account the communities' needs for cultural customization without getting in the way of introducing new technologies is the way to go.