This is a ctOS themed login screen for GDM (and MDM by default). (FOR 1080P SCREENS)
Helvetica LT Std Bold and Roman fonts need to be installed for the theme to appear as intended. Those fonts are included in the package for your convenience.
The session button is hidden within the 'O' in ctOS.
The shutdown/restart/halt menu is the Blume icon in the bottom right.
(The picture shown in here has the Blume logo slightly cutoff because of the screenshot. The actual theme does NOT have the logo cutoff.)
1 - Untar the archive
2 - Install the fonts
3 - Install ctos.tar.gz using your theme manager