UI: Fix toolbar text using theme color

[rev3rsor]

Issue: #14664

Toolbar buttons don't use barTextColor from theme

What I did

Change defaulted text color to theme variable

How to test

Does this need a new example in the kitchen sink apps?

• To test, set barTextColor in theme.create() e.g.

// examples/cra-kitchen-sink/.storybook/manager.js

addons.setConfig({
  ...
  theme: create({
    base: 'light',
    barTextColor: 'green', // <--
    brandTitle: 'CRA Kitchen Sink',
    brandUrl: 'https://github.com/storybookjs/storybook/tree/master/examples/cra-kitchen-sink',
    gridCellSize: 12,
  }),
});

Pre-fix: color only applies to icon buttons
Post-fix: unselected button text should be the same color as well

[shilman]

LGTM! Thanks for the fix @rev3rsor!