FR: ButtonGroup should support "outlined" prop modifier

[DannyDelott]

Environment

• Package version(s): 3.36.0
• Operating System: osx
• Browser name and version: Version 86.0.4240.198 (Official Build) (x86_64)

Background

The ButtonGroup component renders a small amount of margin between the buttons whenever you use outlined buttons in Dark mode.

It looks like this behavior has existed since the outlined prop was introduced in @blueprintjs/core@3.24.0

Code Sandbox

https://codesandbox.io/s/green-sun-tmzq9?file=/src/App.js

Expected behavior

The ButtonGroup should render w/out any margin between buttons (see non-dark mode equivalent)

Possible solution

[adidahiya]

Thanks for pointing this out. ButtonGroup doesn't have outlined support yet; we need to add styling at the group container level just like minimal. It wouldn't make sense to have some buttons in a group outlined while others are not, so the prop & CSS class should be applied to the ButtonGroup.