[material-ui] Attach default light color scheme when enabling CSS theme variables

[siriwatknp]

Issue

According to the docs, the code below should use @media (prefers-color-scheme: dark) for dark color scheme

const theme = createTheme({
  cssVariables: true,
  colorSchemes: { dark: true },
});

The colors are generated for both color schemes but the selector for dark palette is not using @media (prefers-color-scheme).

Fix

Set colorSchemes.light = true if no palette is provided. A test added.

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

[mui-bot]

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