Add convention for CSS custom property usage in common.md

[wpinrui]

Summary

All mockup styles.module.scss files use hardcoded rgba(8, 8, 8, 0.82), rgba(8, 8, 8, 0.85), and rgba(8, 8, 8, 0.75) for panel backgrounds instead of the CSS custom properties (--panel-bg, --panel-bg-inner) defined in globals.scss.

Action

Run -m to add an instruction to common.md telling implementers to use var(--panel-bg) / var(--panel-bg-inner) for container backgrounds — not to copy hardcoded rgba values from the mockups.