docs: function mergeOuterLocalTheme(outerTheme, localTheme) {
    if (typeof localTheme === 'function') {
      const mergedTheme = localTheme(outerTheme);
  
      warning(
        mergedTheme,
        [
          'Material-UI: you should return an object from your theme function, i.e.',
          '<ThemeProvider theme={() => ({})} />',
        ].join('\n'),
      );
  
      return mergedTheme;
    }
  
    return { ...outerTheme, ...localTheme };
  }