Update banner button styles

[mollykreis]

Pull Request

🤨 Rationale

The "dismiss" button in the nimble-banner had the wrong styling in the following ways:

• It was the wrong size. It was 16x16, but it should have been 24x24.
• It was using the wrong icon. It was using the xmark icon, but it should have been using the times icon.
• The hover styling was wrong. It had custom styling to make it look like a ghost button with a fill on hover (not like any of our standard nimble-button appearance modes). Instead, it should be a standard content-hidden, appearance="ghost", appearance-variant="primary" button.

Note: A number of these differences were because the design spec was updated after the banner was initially implemented.

👩‍💻 Implementation

1. Wrapped the contents of the banner's template in a theme provider hard-coded to the color theme
2. Updated storybook docs to specify that a slotted action button should use the primary appearance variant
3. Updated instances of the banner within nimble to set appearance-variant="primary" on slotted buttons

Those three changes allowed me to remove a lot of custom styling from the banner.

🧪 Testing

• Verified that the only chromatic change was due to the "dismiss" button

✅ Checklist

• [ ] I have updated the project documentation to reflect my changes or determined no changes are needed.

[mollykreis]

@m-akinc, will you buddy this PR for me?

[m-akinc]

Note: A number of these differences were because the design spec was updated after the banner was initially implemented.

Thanks for exonerating me! 😁😅