You can find some good gtk themes by searching for theme at gnome-look.org, or deviantart.
Also, you can search for "gtk 3.4 themes" on the net. There are some good ones: Evolve, elementary, Orion, Adwaita-X, Greybird, Ambiance Blue...
Then, download and unzip them (if necessary), and copy them to the folder .themes in your home folder (it is hidden, press Ctrl+H to unhide).
If you want root programs (such as Synaptic, which asks for your password) to use it, run Files as root (open a terminal and type "sudo nautilus"), then copy the themes to File System/usr/share/themes.
Usually these themes also come with a matching window theme, so you can open Cinnamon Settings -> Themes -> Other settings to set the GTK and window theme.