The commands are:
sudo apt update
sudo apt install qt-style-kvantum qt-style-kvantum-themes qt-style-kvantum-l10n
After trying to compile it from Kvantum’s GitHub, I decided to ask an AI instead, and this worked.
Sharing it here to save you some time.
Kvantum in Debian 13 is a theme engine for Qt-based applications that allows applying advanced visual styles using SVG. It mainly affects Qt programs like Dolphin, system settings, or KDE apps, but it does not modify GTK-based applications (such as Firefox, GIMP) nor the overall system appearance outside of Qt. Additionally, its effect depends on whether applications respect the Qt style engine, so some may ignore it partially or completely.