Document breaking changes from Simplify ThemeProvider

[aaronccasanova]

Go through the Simplify ThemeProvider PR and...

• Add relevant changes to unreleased-v8.md
• Add relevant changes to the v7 to v8 migration guide

Note: The above steps were intentionally dismissed in the Simply ThemeProvider PR to unblock other Polaris members working on design tokens.

[aaronccasanova]

(WIP) Going through the PR now and temporarily adding my notes here (In case someone else needs to pick up this issue for whatever reason).

• Removed ThemeProvider from .storybook/polaris-readme-loader
• Added CustomProperties from .storybook/polaris-readme-loader
• Removed the custom theme from the .storybook/preview AppProvider (Now sets a single colorScheme prop)
• Removed the ThemeProvider component from the AppProvider
• Added the CustomProperties component to the AppProvider
• Removed the custom theme prop from the AppProvider
• Added a colorScheme prop to the AppProvider (Forwards to the CustomProperties component)
• Added setBodyStyles method to AppProvider
  • Called on component mount and update
  • Sets the custom color-scheme attribute on the body element
  • Sets inline text color and background color custom properties on the body element
  • Note: This behavior was previously handled by the ThemeProvider component
• Removed ThemeProvider storybook entries from AppProvider README
• Added CustomProperties storybook entries to AppProvider REAME
• Created the CustomProperties component
  • We need to revisit the inline text color styles set in this file. An issue has already been created for this:
• Added a CustomProperties README with a component description and storybook entries
• Added CustomProperties stylesheet utilities (getCustomProperties, getColorSchemeDeclarations`, etc...)
• Added test for the CustomProperties component and all of its utilities
• Removed the ThemeProvider component and useTheme hook from the ContextualSaveBar component
• Added the CustomProperties component to the ContextualSaveBar component with the color scheme explicitly set to dark
• Removed the ThemeProvider from the ContextualSaveBar tests
• Removed the ThemeProvider component from the Toast component
• Added the CustomProperties component to the Toast component with the color scheme explicitly set to dark
• Removed the useTheme hook from the Navigation component
• Removed ThemeContext, ThemeConfig, buildThemeContext, and buildCustomProperties from the PolarisTestProvider component
• Added the CustomProperties component and colorScheme prop to the PolarisTestProvider component
• Removed the ThemeProvider component from the PopoverOverlay component
• Added the CustomProperties component to the PopoverOverlay component
• Deleted the ThemeProvider component, README, storybook entries, and tests
  • Features no longer supported:
    • Inverse colorScheme prop
    • Theme context accessible to children
    • TBD
• Removed the ThemeProvider component from the Search component
• Added the CustomProperties component to the Search component with the color scheme explicitly set to dark
• Removed the ThemeProvider export from polaris-react
• Added the CustomProperties export from polaris-react
• Added color scheme and static tokens to a top level designTokens directory
• Removed all theme utilities in utilities/theme:
  • ThemeContext React context
  • useTheme React hook and tests
  • Theme type
  • ThemeConfig type
  • ProcessThemeConfig type
  • RoleColors type
  • Role type
  • AppThemeConfig type
  • buildCustomProperties utility and tests
  • buildThemeContext utility and tests
  • toCssCustomPropertySyntax utility and tests
  • toString utility and tests

[aaronccasanova]

Resolved in the following PR: #5013