Android comes with loads of designs; most handset manufacturers use their own to 'differentiate' their products, widgets can be themed, and entire components like the launcher and the home screen can be replaced. To say nothing of entirely different UXes like the ones embedded in TVs, e-readers etc.
I'm pretty sure quite a lot of the look of (desktop) Windows can be changed around too - I've just had a look in Control Panel on a machine I have access to and there's a whole 'Appearance and Personalization' section with sub-entries including 'Change the theme'.
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