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
Open GIMP → Edit → Preferences → Themes → gimp-royal