Materia (formerly Flat-Plat) is a Material Design theme for GNOME/GTK+ based desktop environments.
It supports GTK+ 3, GTK+ 2, GNOME Shell, Budgie, Cinnamon, MATE, Unity, LightDM, GDM, Chrome theme, etc.
Also Materia has compatibility with oomox theme designer.
Ranked in these QuestionsQuestion Ranking
Pro Good support for various DEs
Includes support for Gnome Shell, has a Cinnamon desktop theme, plus supports XFCE, Mate and all the usual suspects.
Pro Under continuous active development
Responsive and helpful developer.
Pro Customizable via Oomox app
Users can build completely personal version easily using Oomox Gui app.
Pro Corresponding theme for Qt/KDE
There is a corresponding theme for the kvantum-engine under Qt/KDE
Con No Round Corners
Let's be honest Round Corners is a must in 2020