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...
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...