[EuiComment] Allow comment header to take a styled message color and border

[1Copenut]

Is your feature request related to a problem? Please describe.
@stephmilovic is working on an error styled message in EuiCommentList. She's looking for a way to be able to set a background color and border for errors or other styled messages.

Describe the solution you'd like
Ability to override the background color and border color on individual EuiCommentListItem components.

Additional context
@andreadelrio && @cee-chen discussed this with Steph and I in a Slack thread. The image attached below captures that discussion and gives an idea what the style overrides should produce.

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[cee-chen]

Quick note: the icon buttons in the top right should likely also change colors to match the background

[itzabhinavarya]

Hii @cee-chen , Will you please explain this issue to me,Briefly?
I am a begineer and want to contribute,Please explain this issue.

[cee-chen]

@itzabhinavarya We'd recommend you take a look at the good first issues label instead. This issue is already being worked on by @breehall.

Please also make sure you take a look at our contributing to EUI wiki!