Bump vega-themes from 2.6.1 to 2.7.1
Bump vega-themes from 2.6.1 to 2.7.1

Bumps [vega-themes](https://github.com/vega/vega-themes) from 2.6.1 to 2.7.1.
- [Release notes](https://github.com/vega/vega-themes/releases)
- [Commits](https://github.com/vega/vega-themes/compare/v2.6.1...v2.7.1)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
vega/vega-embed,vega/vega-embed,vega/vega-embed,vega/vega-embed