Increase color contrast between disabled and enabled dates in calendar

[jkaltoft]

Which issue does this PR close?

This PR closes #3414

What is the new behavior?

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Are there any additional context?

Checklist:

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Reminders

• [ ] Make sure you have implemented tests following the guidelines in: "The good: Test".
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Review

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• [ ] Do a self-review.
• [x] Request that the changes are code-reviewed
• [x] 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.