fix(theme): fix useColorMode().colorMode leading to React hydration mismatches
#10954
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Motivation
It's not safe to init
useState(getPersistedColorModeIfBrowser()), and can lead to React hydration mismatches.We notably had one visible in
docusaurus build --devwhen loading a page in dark mode:Using this code is safe and prevents hydration mismatches by using the exact same value that was used during SSG.
Yes the state lags behind, but this is on purpose. Related to #7986
If users need to know the color mode without lag, they can't use that in their React render code or they'll create themselves hydration errors.
If they really know what they are doing and need a fresh value, they can read
document.documentElement.getAttribute('data-theme')directly, but we don't want to encourage that pattern as it's unsafe to do so.Test Plan
CI + preview
Test links
https://deploy-preview-10954--docusaurus-2.netlify.app/