@nibzy26 Please tone down the language and contain your frustration. This is how it works in software in general and Open Source in particular. I am not saying it is ideal, but it is what it is.
Wanting to dive in and do something that no one else has done before, you cannot expect to have the way already paved and documented for you. You are pioneering.
As to why icons/Humanity/mimes/48? Possibly because this is a fallback if the icons/suru folder does not contain the relevant files.
If it is the case how I have described it, then that is how theming works in general on Linux, I have never looked for documentation on the matter, but I am sure the information is out there.