Question about windows, NOT "Windows"

I am a programing newbie (started with Python here) so forgive my ignorance but I have a question.
I’m sure it’s probably way out of my league because no one has done it that I can tell, but here goes.

If I want to modify a window anything opens in, how complicated would that be? Is there a specific program that tells an operating system “windows are rectangles and they all look and function like this.”

By modify, I mean: Just say I open Notepad. It opens in a rectangle. What if I want it to open in an oval with a full border around it that contains an area that can contain Notepad’s controls in button form? Or the border could contain a color graphic that moved around the border that could tell how much time you’ve spent using “Notepad”

I can’t find anyone doing anything like this. What exactly would I need to know to begin to make this happen? I’m assuming gamers would love this too if it didn’t create lag or eat gobs of system resources. if you wanted a futuristic desktop theme your windows could look futuristic rather than just the standard window we all know, that hasn’t seemed to change since the beginning. Computer manufacturers like Alienware could make alien based looking stuff. MSI could incorporate their dragon. If it was open source somewhere people could make their own.

If anyone knows of something like this can you point me to it? If I type window or windows into a search engine question it shows a million and a half suggestions for Microsoft Windows, and my question has nothing to do with Microsoft specifically.

The reason people dont try this is because the window is specific to the operating system. What this means is that all usual programs refer to a standard window, built in the api they are using. This makes so the programmer does not have to make/design his own window.
Thats why for example the program “notepad” has a diffrent window design for windows than it does on mac-os.

It is not really possible to change this for a single program since they are just reffering to the window microsoft uses standard. You would need to change this inside windows (this is why most google posts come down to what you found). I would think this will change it for all programs that use this window and not just the program you want it to. (I have no experiance on changing a window design tho.)