I'm not sure, I found it a while back in one of these slow days so I was browsing online and found this and a few others. I really should've written down the source.
What's interesting is that in Gnome 44 the same rounder corners extension worked just fine. I'm not too sure if last time I used Fedora it was on Gnome 43 or perhaps it had already updated to Gnome 44, but I seem to remember this worked fine as well.
Remember to also enable GTK themes under Appearance/Application Style/
and use this button:
Then choose it in the dropdown:
This will help with things like about-this-mc and other Gnome-based apps. For instance, I do not use Dolphin in KDE. Still use Gnome Files app.
If Gnome desktop is already installed then it should be available. If you do not see it in the KDE applications menu, then run it by doing this in Terminal:
nautilus
Otherwise:
sudo apt install nautilus
Mystery solved: I installed the wrong extension!
Still, I have to agree it makes no sense for some windows to be rounded by default while others are a mix of some corners rounded or not at all...
Awesome. That is much better than thinking there is no practical solution available. 
I too have that wallpaper.I got it from here.
I usually search for 'flat wallpapers/vector wallpapers' on google to get similar results.
Inspired by my old Ubuntu VM layout I see. (Apologies if I offended you - I was just pointing out).
No worries, you didn't offend me in any way
But more than inspiration, I'd say this is the result of discovering what's gnome extensions, how to add them, and then combining every single extension that I liked and felt useful to me
I deleted my screenshot and following comments replies as i didn't appreciate the comments it took the wind out of my sails. I don't need to be questioned about the icons i'm using saying i could get in trouble, i don't release any of it i just use for my personal use. Have a nice day.
It would seem like the written word would be a better record than spoken, yet the written word seems to be the most prone to lead to misunderstandings.
In the post in question:
Turtle is referring to how replacing Symbolic Links for Icon sets can cause a misread for icon files resulting in an icon displaying as "File Missing" instead of as the pictographic icon.
Since I make icon sets and distribute them, including that I create and include the Symlinks to complete the sets, my familiarity with the subject probably has a great deal to do with how easily I understood the intended meaning.
He did not mean "get into trouble" with an authority, but "have an issue" with files.
On a side note, the icon set you referenced is Free Open Source and you can mix and match and release them at will if you wanted to.
The tone I got from Turtle's post was that he was impressed by your creativity and expression. Kind of a "Wow, you were able to do that? I have run into difficulties trying to do that." And again, I think that this reading on my part is due to the fact that so many people are impressed. Your posts in this thread constantly bring refreshing, welcome and dazzling contributions that stretch beyond just simple appearance settings or aesthetics - but delve into the core of our humanity that makes this such a great place.
I am truly sorry for misunderstanding. I have had a rough week mentally. I meant no harm to @Turtle11 or anyone and i sincerely apologize for misunderstanding and thank you @Aravisian that's very kind of you. sorry fam.
I really like this one. Maybe you could try to mirror the image so that the dark sky is on the left, so your icons would be more readable. Just a thought.
The McMojave Circle / Wings Icons mix i am using here is for personal use. How i did this was by copying images in Wings icons places and pasting them in McMojave place overriding current images in folder. I hope this properly explains for everyone.
