Just a heads up that the Materia (formerly Flat-Pat) GTK theme has recently gained (with a little input from me ) Cinnamon theme support.
Materia - https://github.com/nana-4/materia-theme
There is a PPA but it is unlikely to have been updated with the Cinnamon theme support as yet.
In the meantime to install directly from source...
wget https://github.com/nana-4/materia-theme/archive/master.zip
unzip master.zip
cd materia-theme-master
sudo ./install.sh
Select your preferred variant of the Materia theme in the Cinnamon Themes module for Desktop, Controls and Windows Borders.
Installing Roboto font is optional but recommended.
apt install fonts-roboto
Regular and Dark variants in action...