[core] Replace CssVarsProvider with ThemeProvider

[siriwatknp]

To test mui/material-ui#43115 and get feedback from Toolpad team.

Context

The core team is merging the CssVarsProvider and extendTheme into ThemeProvider and createTheme to prevent the confusion of using the APIs. The detail is in this issue.

Changes

• Remove the CssVarsProvider because now ThemeProvider can handle with/without colorSchemes node.
• I see that Toolpad has a custom logic to receive a theme with { light: Theme, dark: Theme } structure so this is untouched.
• The dualTheme can be checked by reading { allColorSchemes } = useColorScheme() if allColorSchemes.length > 1.

For more detail of the implementation, see the Material UI PR: mui/material-ui#43115

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[apedroferreira]

Having allColorSchemes seems good to me if we plan to support any number of color schemes with any names.

Since CSS vars themes are becoming standard I wouldn't even support an object with { light: Theme, dark: Theme } in Toolpad anymore. Users would need to use a CSS vars theme if they wanted dual theming / a theme switcher and this way they would never be surprised by running into theme flash issues when server-side rendering, for example. I guess if we did this the implementation would be pretty straightforward?

[siriwatknp]

Since CSS vars themes are becoming standard I wouldn't even support an object with { light: Theme, dark: Theme } in Toolpad anymore

Yes, if that's the case the AppThemeProvider can be removed. It will be much simpler.

[Janpot]

@siriwatknp I updated the implementation a bit to make sure useColorScheme is never called when there is a v5 theme.

Verified that the implementation works against v5

[Janpot]

Everything labeled as "next" will eventually get a stable v6 specifier

[apedroferreira]

This looks great, thanks!

[apedroferreira]

If this selector needs to be added every time I guess we could override it ourselves, or make it fully configurable. As right now I guess the AppProvider theming wouldn't work with any other selector.