OS X doesn't really like to be themed, so something like that should not be attempted by someone without knowing EXACTLY what they're doing.
You should just install Ubuntu on your machine, the latest update is splendid.
it really annoys me when people don't answer the question and preach what you should or shouldn't do instead.
If you really want to share your own opinions, do it at your own thread, or in one that asks about it. On the other hand, if the question asked is about "how to..." then your answer is really not answering anything. If you did reply the actual question, then sure, you can add an extra to the end, as some sort of btw, or a propos. Otherwise it makes life a whole lot harder for people like me, who are looking for answers to specific questions, and not opinion exchanges.
OK, anyways, so far I did icon replacement manually. I used Ubuntu to convert some of default icons from PNG format into ICNS format using png2icns tool from the icnsutils package: sudo apt-get install icnsutils.
Then, I searched for all the *.icns files on the mac under: /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/
I replaced some of them with my custom made ones. That is for starters. Still looking for a way to alter colors. I'm altering OSX High Sierra, btw.