[system] Add `defaultMode` to `InitColorSchemeScript`

[siriwatknp]

closes #43972

Root cause

InitColorSchemeScript has a hard-coded default mode as system when the user first visit to the site, however this contradict the flow if the project has <ThemeProvider defaultMode="light"> on user's dark environment.

Solution

• Add defaultMode to InitColorSchemeScript.
• Update docs to make sure that both places are defined for custom defaultMode.

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• [x] I have followed (at least) the PR section of the contributing guide.

[mui-bot]

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• docs/data/material/customization/dark-mode/dark-mode.md

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[oliviertassinari]

@siriwatknp what do you think about my previous comments?