$ sudo apt-get update
$ sudo apt-get install k3b-extrathemes 

k3b-extrathemes is installed in your system.

Make ensure the k3b-extrathemes package were installed using the commands given below,

$ sudo dpkg-query -l | grep k3b-extrathemes *

You will get with k3b-extrathemes package name, version, architecture and description in a table

k3b-extrathemes package basic information: