COSMIC now supports theming GTK3/4 applications

7 people according to the what System76 CEO said on a interview

It is possible. This is the beginning of our theme integration support.

It is a desktop environment, which is the entire graphical user interface of the OS, and all of its bundled applications. It is also a platform which developers can build COSMIC applets and applications with. Applications being desktop applications, and applets being the shell components. Shell components are small interface elements such as the panel, dock, panel buttons, on-screen displays, launcher, etc.

It will take its place alongside the two giants in this space: KDE and GNOME. KDE being a desktop environment whose applets and applications are written in C++/JavaScript using Qt/QML as its GUI library. GNOME being and desktop environment whose applets are written in JavaScript with the GNOME Shell Toolkit; and its applications are written primarily in C with GTK as its GUI library.

COSMIC is instead built from the ground up entirely in Rust from top to bottom. Every applet and application is written in Rust, and the same libcosmic GUI library is used for developing both of them. Rust is a statically typed programming language which has dethroned C/C++ in recent years, and has been the most loved programming language on StackOverflow for the last eight years. We aim to make COSMIC the preferred platform for developing applications in Rust, with a GUI toolkit that's easier to develop than the alternatives.

Not yet, but PRs welcome if anyone has experience with theming Qt.