Flat Remix GNOME theme is a pretty simple shell theme inspired on material design. It follows a modern design using "flat" colors with high contrasts and sharp borders.
Run gsettings set org.gnome.shell.extensions.user-theme name "Flat-Remix" or select "Flat-Remix" via gnome-tweaks.
Terminal installation
cd /tmp && rm -rf flat-remix-gnome &&
git clone https://github.com/daniruiz/flat-remix-gnome &&cd flat-remix-gnome;# If you want to change the login wallpaper# (cd src && ./build.sh --login-background /path/to/image)
sudo make install