adds background-color to compressed button group

[ryankeairns]

Fixes #2564

Summary

Adds background color to compressed button groups to prevent underlying background from showing through. Also, reduced the tint on 'selected' button background color for a touch more contrast.

As you'll see below, these minor changes result in the button groups also aligning better to the styles of the other compressed inputs.

Before (left) and After (right)

With Canvas sidebar background color

Checklist

• Checked in dark mode
• Checked in mobile
• Checked in IE11 and Firefox
• Props have proper autodocs
• Added documentation examples
• Added or updated jest tests
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[cchaos]

Looks a lot better and even the dark mode selection is more visible. Just had a minor comment but really it's good to go.