@sierdzio

It worked as you said.

Thank you for your help.

I wish that it was possible to put different icons into a resource file, to be able to call upon any of them at a time.

I put the ico file into my project folder and called it from .pro (project) file with that single line of code.
It wasn't necessary to call any sort of function of window decoration (perhaps I haven't observed the issue yet). However, I believe that it was necessary to convert that file into .ico, what is ...

well... the whole point of SVG is scalability, so that they look the same on any sort of device. I don't understand why Microsoft would stick to that .ico

"...ICO files contain one or more small images at multiple sizes and color depths, such that they may be scaled appropriately."

Perhaps it makes sense in some way. I simply spent more time poking around HTML 5.