How to Change Themes with the Operating System in a Qt App

When creating a Qt app, changing themes involves understanding how the operating system's light or dark mode settings interact with the app's theme. Initially, the author tested light mode in Packet Sender, a messaging app developed by Dan Nagle, who only uses dark mode personally. This led to a bug where some dark mode theme elements remained even after switching to light mode.

To fix this, the app needed to manually test the UI going from dark to light mode. It was discovered that Packet Sender followed the operating system's theme settings, but there was an additional setting built into the app that could be toggled for manual testing. After switching the operating system's mode, Packet Sender would read the new setting and apply the correct theme upon launch.

However, the app did not update itself when the OS changed themes automatically based on time or light conditions. Dan suggested using a thread to solve the problem, but the solution using a QThread crashed. It was realized that listening for light/dark changes was a GUI-state problem, not a background-compute issue. The optimal solution was to use a QTimer on the main thread, which runs in the same thread as the widgets and serializes the process with theme events.

This approach is safe on macOS and avoids race conditions. Ultimately, a function named setupThemePolling() was created to continuously poll the system's theme and apply the appropriate theme to the app, ensuring a smooth user experience without race conditions.