Royal-Gtk is a modern flat theme with a combination of light and dark elements based on Numix. According to the developer of the theme, he modified the theme Numix to make a better feeling from using it, because it was too dark. He did it to himself, but then shared their work with the community.
Now this theme contains less dark colors, dark toolbar for GTK3 apps, dark theme for GIMP and Qt Creator, new window controls (similar to those used in the operating system OS X 10.10). The developer modified the theme only for Ubuntu Unity, but as far as I checked, it can also work under Linux Mint. If you are using QtCreator, you can improve its appearance by opening the Qt-SDK from the Menu of your distribution, the label on the skin is installed together with the theme.
Installation into Linux Mint:
sudo add-apt-repository ppa:noobslab/themes
sudo apt-get update
sudo apt-get install royal-gtk-theme[/simterm]
Open GIMP → Edit → Preferences → Themes → gimp-royal