The package can now be found on MELPA.
You can install in emacs with M-x list-packages RET and find caroline theme.
Add this right here to your .emacs config file for a manual install.
I have not put the package on MELPA, but considering it for a later time...
You'll have to install manually.
Add this right here to your .emacs config file.
This will have the theme load during emacs startup.
Download caroline-theme.el to the directory ~/.emacs.d/themes/