Just extract the package to ~/.kde/share/apps/desktoptheme/
Download Caledonia theme.
tar -xvvf ~/Downloads/Caledonia.tar.gz -C .kde/share/apps/desktoptheme/
Now, visit the Desktop theme menu to set this as default.
Login screen theme:
Apparently there is no interface to change these, for regular users. However, the location is:
Joel G Mathew, known in tech circles by the pseudonym Droidzone, is an opensource and programming enthusiast.
His favorite pastime is grappling with GNU compilers, discovering newer Linux secrets, writing scripts, hacking roms, and programs (nothing illegal), reading, blogging. and testing out the latest gadgets.
When away from the tech world, Dr Joel G. Mathew is a practising ENT Surgeon, busy with surgeries and clinical practise.