💄 style: colour scrollbar to match primary colour

[welpo]

Summary

Added scrollbar-color CSS variable to style scrollbars consistently with the theme's primary color. Respects the use of custom CSS / skins.

Related issue

Same goal as #429: theme cohesion.

Changes

Added scrollbar-color: var(--primary-color) transparent; to the global styles for custom scrollbar styling that matches the theme.

Screenshots

Type of change

• Bug fix (fixes an issue without altering functionality)
• New feature (adds non-breaking functionality)
• Breaking change (alters existing functionality)
• UI/UX improvement (enhances user interface without altering functionality)
• Refactor (improves code quality without altering functionality)
• Documentation update
• Other (please describe below)

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Checklist

• I have verified the accessibility of my changes
• I have tested all possible scenarios for this change
• I have updated theme.toml with a sane default for the feature
• I have made corresponding changes to the documentation:
• Updated config.toml comments
• Updated theme.toml comments
• Updated "Mastering tabi" post in English
• Updated "Mastering tabi" post in Spanish
• Updated "Mastering tabi" post in Catalan

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