Add dark mode to the built-in PDF preview

What feature would you like to see?

Please add dark-mode support to the built-in PDF preview in the Codex/ChatGPT desktop app.

Problem

The rest of the application can use a dark theme, but opening a PDF displays bright white pages. This creates a harsh visual transition and is uncomfortable during long coding or research sessions, especially when reading papers, technical documentation, or specifications at night.

Proposed behavior

• Follow the app theme automatically by default.
• Add a PDF-viewer option for Light / Dark / System.
• Remember the selected preference across sessions.
• Prefer a rendering approach that darkens page backgrounds and text without unnecessarily inverting figures, screenshots, or images.

A basic color-inversion mode would already be useful, but a document-aware dark mode would be better where feasible.

Why this matters

PDFs are commonly opened alongside code during research and development. A dark PDF preview would make the desktop workflow visually consistent and reduce eye strain.

Platform

Windows desktop app, currently branded ChatGPT (previously Codex).