Allow flag notification colors to be overridden

[jkaltoft]

Use CSS custom property instead of color value to make it possible.

Fixes #2353

Which issue does this PR close?

This PR closes # (insert issue number here)

What is the new behavior?

success and warning notification colors for <kirby-flag> can be overridden in consumer projects.

Does this PR introduce a breaking change?

• Yes
• No

Are there any additional context?

danger notification color for flags is a special case and cannot be controlled by consumer project by overriding the custom property --kirby-danger .

Checklist:

The following tasks should be carried out in sequence in order to follow the process of contributing correctly.

Reminders

• Make sure you have implemented tests following the guidelines in: "The good: Test".
• Make sure you have updated the cookbook with examples and showcases (for bug fixes, enhancements & new components).

Review

• Do a self-review.
• Request that the changes are code-reviewed
• Request that the changes are UX reviewed (only necessary if your PR introduces visual changes)

When the pull request has been approved it will be merged to develop by Team Kirby.