Polish: Make code-block scrollbar styling fully theme-aware

[hdcodedev]

Summary

Make scrollbar styling theme-aware and consistent with the design system.

Why

Current code block scrollbars are hardcoded dark-toned values and are not fully aligned with future light theme support.

Scope

• Replace hardcoded scrollbar colors with CSS variables.
• Define both dark and light theme values.
• Ensure appearance works in WebKit and Firefox fallbacks.

Acceptance Criteria

• Scrollbar styles read from theme tokens.
• Light theme scrollbars are legible and visually coherent.
• No regressions in code block usability.

[hdcodedev]

Closed — addressed or superseded by modernization work in #44.