Bump @storybook/theming from 6.1.17 to 6.1.21

Bumps [@storybook/theming](https://github.com/storybookjs/storybook/tree/HEAD/lib/theming) from 6.1.17 to 6.1.21.
- [Release notes](https://github.com/storybookjs/storybook/releases)
- [Changelog](https://github.com/storybookjs/storybook/blob/next/CHANGELOG.md)
- [Commits](https://github.com/storybookjs/storybook/commits/v6.1.21/lib/theming)

Signed-off-by: dependabot[bot] <support@github.com>