[System] Simplify theme input types for CssVarsProvider

[siriwatknp]

widening theme prop typings in CssVarsProvider:

<CssVarsProvider theme={
 // this can be any object with { cssVarsPrefix, colorSchemes } because they are the only thing that `CssVarsProvider` cares about
}>

Joy

• rename ThemeInput to CssVarsThemeOptions to follow Material UI

Material UI

• add missing type for snackbar color
• simplify module augmentation to ease maintenance

Module augmentation still works: https://codesandbox.io/s/create-react-app-with-typescript-forked-ji3dgf?file=/src/App.tsx

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

[mui-bot]

No bundle size changes

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