This doesn't exist as a built-in feature. But you can use alternate themes that other people have designed. Search for theme on Stackapps. In particular, you may like MinimalOverflow (beware, it hasn't been updated in a while so it may be missing some elements), or StackExchange Theme Switcher if there's some other Stack Exchange site design that you really like.
Gilles 'SO- stop being evil'
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