KDE Oxygen widget theme for GTK
Oxygen-Gtk is a port of the default KDE widget theme (Oxygen) to GTK. Its primary goal is to ensure visual consistency between GTK and Qt-based applications running under KDE. A secondary objective is to also have a stand-alone nice looking GTK theme that behaves well in other desktop environments.
Homepage: https://projects.kde.org/projects/playground/artwork/oxygen-gtk/