docs: Increase sidebar text contrast in dark mode

[wbinnssmith]

This increases the contrast of the sidebar text against its dark background by slightly lightening it. It now passes WCAG AA guidelines.

Test Plan: https://webaim.org/resources/contrastchecker/

[vercel]

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Name	Status	Preview	Comments	Updated
turbo-site	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Feb 3, 2023 at 5:26PM (UTC)

9 Ignored Deployments

Name	Status	Preview	Comments	Updated
examples-basic-web	⬜️ Ignored (Inspect)			Feb 3, 2023 at 5:26PM (UTC)
examples-cra-web	⬜️ Ignored (Inspect)			Feb 3, 2023 at 5:26PM (UTC)
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examples-native-web	⬜️ Ignored (Inspect)			Feb 3, 2023 at 5:26PM (UTC)
examples-nonmonorepo	⬜️ Ignored (Inspect)			Feb 3, 2023 at 5:26PM (UTC)
examples-svelte-web	⬜️ Ignored (Inspect)			Feb 3, 2023 at 5:26PM (UTC)
examples-tailwind-web	⬜️ Ignored (Inspect)			Feb 3, 2023 at 5:26PM (UTC)
examples-vite-web	⬜️ Ignored (Inspect)			Feb 3, 2023 at 5:26PM (UTC)

[github-actions]

🟢 CI successful 🟢

Thanks

[arlyon]

Seems reasonable to avoid forking the theme. Much more readable :)