[primer/react] Update documentation on theme overrides

[cbix]

Since primer/react v37, most components use CSS variables instead of theme tokens. The documentation only mentions overriding values in the theme object, however this doesn't work anymore for certain colors:

const customTheme = merge(theme, {
  colorSchemes: {
    light: {
      colors: {
        btn: {
          primary: {
            bg: '#cccc00',
          },
        },
      },
    },    
  },    
});
const App = () => (
  <ThemeProvider theme={customTheme}>
    <BaseStyles>
      <Button variant="primary">Hello</Button>
    </BaseStyles>
  </ThemeProvider>
);

Expected:

Result:

The documentation should be updated so users know how to override theme values in primer/react v37.