Adding a PPA is an action that requires trust.
If I haven't read the source code of the repository that is held in the PPA, I shouldn't install a random PPA for security reasons.
Therefore, is there a way to install a theme (I'm interested in a dark one) for Ubuntu 16.04 without adding a PPA? I've already tried to download this one, and after decompressing the tar.gz and looking that there are no executable files, I've placed it in my ~/.themes folder but unity-tweak-tool doesn't see it (maybe this tool is deprecated for this version of ubuntu?).