fix: Fine tune error bar marker color

[jperals]

Description

Related docs: 3T8mA9G0PI3D, JaIkAMOgChPb.

How has this been tested?

• Manually in deployed error bars dev page (light and dark mode)
• Visual regression tests show expected diff

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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