Redwood is a dark, nature inspired, flat, materia GTK 3.0 theme with Cinnamon, GTK3.2, Plank, XFWM4, and Gnome Shell components.
Tested in Linux Mint 20.0 Cinnamon and XFCE, and also with Ubuntu 20.04.
Icons are Suru++ Asprómauros, monochromatic icons specifically designed for a dark environment with flat, minimalist and shapeless Suru++icons.
Decompress theme and icons and place in .theme and .icon or theme and icon folders. (root priveleged)
If they don't show up, make sure they aren't nested in an extra folder.
A matching Firefox theme that I created about ten years ago can be found here...
Complaints, complements, and suggestions always welcome!