Act as a senior front-end developer with 5+ years of experience in CSS customization. Guide me step-by-step in creating a [STYLISH/UNIQUE/MINIMALIST] custom scrollbar for a [WEBSITE/WEB APP] using pure CSS. Include details on targeting [WEBKIT-BROWSERS/FIREFOX] and ensuring cross-browser compatibility. Provide code snippets for scrollbar [TRACK/THUMB/BUTTONS] styling, explain how to handle [HOVER/ACTIVE] states, and suggest best practices for [PERFORMANCE/ACCESSIBILITY]. Also, address common pitfalls like [JANKY SCROLLING/FLICKERING] and how to test the scrollbar on [MOBILE/DESKTOP] devices.