$ sudo apt-get update
$ sudo apt-get install gtk2-engines-qtcurve 

gtk2-engines-qtcurve is installed in your system.

Make ensure the gtk2-engines-qtcurve package were installed using the commands given below,

$ sudo dpkg-query -l | grep gtk2-engines-qtcurve *

You will get with gtk2-engines-qtcurve package name, version, architecture and description in a table

gtk2-engines-qtcurve package basic information: