[docs] fix syntax highlighting on FAQ page

[geoffrich]

Fixes #3764
Fixes sveltejs/sites#295

This seemed to happen because we copied styles from svelte.dev's FAQ page, which is white text on black without syntax highlighting.

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Latest commit: daf592c

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[benmccann]

can we also change the svelte.dev FAQ to match? the whole svelte.dev site used to use dark code blocks, but no longer does and it looks like the FAQ wasn't updated when the docs were