fix(theme-management): Use color library for ThemeColorPickerField

[Kohila]

Summary

This PR installs the library color (https://www.npmjs.com/package/color) into the plugin. The color conversion methods in this libary are used to convert any supported color formats into hex values for the react-colorful UI element.

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Checklist

• Code changes follow repository conventions
• Tests added or updated
• Documentation updated
• Lockfile updated (if package.json changed)

Lockfile note: If you changed package.json, make sure to run pnpm install locally and include the updated pnpm-lock.yaml in this PR so CI's install step succeeds.