Diehard4 is a dark color theme and desktop applications.
Sorry for the spelling and phonetic errors in the text.
Owner and main writer of MintGuide.org is not a native English speaker.
The Diehard4 theme was created on DevianArt developer-designer under the name Zomby specifically for elementary OS Freya, but it successfully installs and runs on other desktop environments such as GNOME, Cinnamon, Unity, Pantheon, Openbox, XFCE, Pekwm and Gala.
Here is the Diehard4 in different Linux Mint desktop environments:
Installation latest version into Linux Mint:
Download the latest version from the developer's page
Unpack downloaded archive in the folder /home/USER/.themes
It remains to choose the theme as shown in the screenshots above.