How to Manually Install New Cinnamon Themes (Linux Mint/Ubuntu)
To manually install new themes for cinnamon from the page provided by Cinnamon Settings, first backup the data of the original theme by opening a terminal and entering
sudo cp -R /usr/share/cinnamon/theme /usr/share/cinnamon/theme_backup
Then download and unpack another theme from http://cinnamon-spices.linuxmint.com/themes, save the files to some location - I will save them on my desktop.
Then you have to copy the data from the unpacked folder to the theme folder of cinnamon:
sudo cp -f /home/howtoforge/Desktop/unpackedfolder/cinnamon/* /usr/share/cinnamon/theme
The new data has then replaced the old. To restore the default theme, copy back the backed up data.