fix(v2): set theme color mode for SSR

[lex111]

Motivation

Based on "bug report" from Discord chat.

Indeed, since the user defines site preferred color mode in config file, we can take this information to set the color mode (used by useTheme) during site build.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Try use colorMode.defaultMode with dark value and build the site.

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

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Size Change: 0 B

Total Size: 564 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	87.3 kB	0 B
website/build/assets/js/main.********.js	390 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	60.7 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	25.6 kB	0 B

_{compressed-size-action}

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Deploy preview for docusaurus-2 ready!

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⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	92
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4383--docusaurus-2.netlify.app/classic/

[slorber]

seems like a little improvement

but I think what the user does is kind of an antipattern and he should rather use css for theming to avoid FOUC