ThemeEngine

ThemeEngine is a macOS‑only utility that lets users open, modify, and rebuild compiled .car files, the binary assets used for system and application them. The program provides a graphical interface for inspecting the contents of these files and applying changes, enabling custom visual styles on supported OS versions. It supports two branches: one targeting macOS 10.11 and later, and another for macOS 11.0 and later, reflecting the differences in the .car format across releases.

The tool is aimed at developers, power users, and designers who need to customize the appearance of macOS components or third‑party apps that rely on .car resources. By handling the low‑level binary format, it removes the need for manual editing of the underlying data structures, streamlining the theming workflow.

For recent macOS releases such as Big Sur, the README outlines additional steps required to edit system .car files, including disabling System Integrity Protection, mounting the sealed system volume in a writable state, and creating a snapshot after modifications. These instructions ensure that users can safely apply changes to protected system resources.