It is a new modern theme for the nice and powerful bug tracker MantisBG http://www.mantisbt.org/
Just put the default.css file to the /css folder of the MantisBG folder. The existed default.css file may be renamed to default.css.orig. Checked with MantisBT 1.2.17.
For discussion and screenshots, please, visit http://www.mantisbt.org/forums/viewtopic.php?f=4&t=22078